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- Abbeville County, South Carolina
- Abert’s squirrel
- Acadia, Louisiana
- Acadia Parish, Louisiana
- Accomack County, Virginia
- Ada County, Idaho
- Adair County, Iowa
- Adair County, Kentucky
- Adair County, Missouri
- Adair County, Oklahoma
- Adams County, Colorado
- Adams County, Idaho
- Adams County, Illinois
- Adams County, Indiana
- Adams County, Iowa
- Adams County, Mississippi
- Adams County, Nebraska
- Adams County, North Dakota
- Adams County, Ohio
- Adams County, Pennsylvania
- Adams County, Washington
- Adams County, Wisconsin
- Addison County, Vermont
- Agile kangaroo rat
- Aiken County, South Carolina
- Aitkin County, Minnesota
- Alabama
- Alachua County, Florida
- Alamance County, North Carolina
- Alameda County, California
- Alamosa County, Colorado
- Alaska
- Alaska marmot
- Albany County, New York
- Albany County, Wyoming
- Albemarle County, Virginia
- Alcona County, Michigan
- Alcorn County, Mississippi
- Aleutians East Borough, Alaska
- Aleutians East Census Area, Alaska
- Aleutians West, Alaska
- Aleutians West Census Area, Alaska
- Alexander County, Illinois
- Alexander County, North Carolina
- Alexandria, Virginia
- Alexandria city, Virginia
- Alexandromys
- Alexandromys oeconomus
- Alfalfa County, Oklahoma
- Alger County, Michigan
- Allamakee County, Iowa
- Allegan County, Michigan
- Allegany County, Maryland
- Allegany County, New York
- Alleghany County, North Carolina
- Alleghany County, Virginia
- Allegheny County, Pennsylvania
- Allegheny woodrat
- Allen, Louisiana
- Allen County, Indiana
- Allen County, Kansas
- Allen County, Kentucky
- Allen County, Ohio
- Allen Parish, Louisiana
- Allendale County, South Carolina
- Allen’s chipmunk
- Alpena County, Michigan
- Alpine County, California
- Alpine chipmunk
- Amador County, California
- Amelia County, Virginia
- American beaver
- American pygmy shrew
- American water shrew
- Amherst County, Virginia
- Amite County, Mississippi
- Ammospermophilus
- Ammospermophilus harrisii
- Ammospermophilus interpres
- Ammospermophilus leucurus
- Ammospermophilus nelsoni
- Anchorage, Alaska
- Anchorage Municipality, Alaska
- Anderson County, Kansas
- Anderson County, Kentucky
- Anderson County, South Carolina
- Anderson County, Tennessee
- Anderson County, Texas
- Andrew County, Missouri
- Andrews County, Texas
- Androscoggin County, Maine
- Angelina County, Texas
- Anne Arundel County, Maryland
- Anoka County, Minnesota
- Anson County, North Carolina
- Antelope County, Nebraska
- Antrim County, Michigan
- Apache County, Arizona
- Aplodonte
- Aplodontia
- Aplodontia rufa
- Aplodontiidae
- Appanoose County, Iowa
- Appling County, Georgia
- Appomattox County, Virginia
- Aransas County, Texas
- Arapahoe County, Colorado
- Arborimus
- Arborimus albipes
- Arborimus longicaudus
- Arborimus pomo
- Archer County, Texas
- Archuleta County, Colorado
- Arctic ground squirrel
- Arctic shrew
- Ardilla de Arizona
- Ardilla de Nayarit
- Ardilla de la costa oeste
- Ardilla de montaña dorada
- Ardilla de tierra canosa
- Ardilla de tierra cola redonda
- Ardilla de tierra de Idaho
- Ardilla de tierra de Washington
- Ardilla de tierra de Wyoming
- Ardilla de tierra de desierto de Mohave
- Ardilla de tierra de la Columbia Británica
- Ardilla de tierra de las montañas Uinta
- Ardilla de tierra de las planicies
- Ardilla de tierra de las praderas
- Ardilla de tierra del oeste
- Ardilla de tierra del pueblo Piute
- Ardilla de tierra del ártico
- Ardilla de tierra gris
- Ardilla de tierra manchada, juancito manchado
- Ardilla gris del este
- Ardilla gris del oeste
- Ardilla listada Sonorense
- Ardilla listada Texana
- Ardilla listada del Valle de San Joaquin
- Ardilla manchada de trecelíneas
- Ardilla manchada texana
- Ardilla orejas peludas
- Ardilla roja
- Ardilla voladora del norte
- Ardilla voladora del sur
- Ardilla zorra del este
- Ardillón común
- Ardillón de California
- Ardillón de California del norte
- Arenac County, Michigan
- Arizona
- Arizona cotton rat
- Arizona gray squirrel
- Arizona pocket mouse
- Arizona shrew
- Arizona woodrat
- Arkansas
- Arkansas County, Arkansas
- Arlington County, Virginia
- Armstrong County, Pennsylvania
- Armstrong County, Texas
- Aroostook County, Maine
- Arthur County, Nebraska
- Arvicolinae
- Ascension, Louisiana
- Ascension Parish, Louisiana
- Ashe County, North Carolina
- Ashland County, Ohio
- Ashland County, Wisconsin
- Ashley County, Arkansas
- Ashtabula County, Ohio
- Asotin County, Washington
- Assumption, Louisiana
- Assumption Parish, Louisiana
- Atascosa County, Texas
- Atchison County, Kansas
- Atchison County, Missouri
- Athens County, Ohio
- Atkinson County, Georgia
- Atlantic County, New Jersey
- Atoka County, Oklahoma
- Attala County, Mississippi
- Attwater’s pocket gopher
- Audrain County, Missouri
- Audubon County, Iowa
- Auglaize County, Ohio
- Augusta County, Virginia
- Aurora County, South Dakota
- Austin County, Texas
- Autauga County, Alabama
- Avery County, North Carolina
- Avoyelles, Louisiana
- Avoyelles Parish, Louisiana
- Baca County, Colorado
- Bacon County, Georgia
- Bailey County, Texas
- Bailey’s pocket mouse
- Baiomyini
- Baiomys
- Baiomys taylori
- Baird’s pocket gopher
- Baja california pocket mouse
- Baker County, Florida
- Baker County, Georgia
- Baker County, Oregon
- Baldwin County, Alabama
- Baldwin County, Georgia
- Ballard County, Kentucky
- Baltimore, Maryland
- Baltimore County, Maryland
- Baltimore city, Maryland
- Bamberg County, South Carolina
- Bandera County, Texas
- Banks County, Georgia
- Banner-tailed kangaroo rat
- Banner County, Nebraska
- Bannock County, Idaho
- Baraga County, Michigan
- Barber County, Kansas
- Barbour County, Alabama
- Barbour County, West Virginia
- Barnes County, North Dakota
- Barnstable County, Massachusetts
- Barnwell County, South Carolina
- Barren County, Kentucky
- Barren ground shrew
- Barron County, Wisconsin
- Barrow County, Georgia
- Barry County, Michigan
- Barry County, Missouri
- Bartholomew County, Indiana
- Barton County, Kansas
- Barton County, Missouri
- Bartow County, Georgia
- Bastrop County, Texas
- Bates County, Missouri
- Bath County, Kentucky
- Bath County, Virginia
- Baxter County, Arkansas
- Bay County, Florida
- Bay County, Michigan
- Bayfield County, Wisconsin
- Baylor County, Texas
- Beach vole
- Beadle County, South Dakota
- Bear Lake County, Idaho
- Beaufort County, North Carolina
- Beaufort County, South Carolina
- Beauregard, Louisiana
- Beauregard Parish, Louisiana
- Beaver County, Oklahoma
- Beaver County, Pennsylvania
- Beaver County, Utah
- Beaverhead County, Montana
- Becker County, Minnesota
- Beckham County, Oklahoma
- Bedford County, Pennsylvania
- Bedford County, Tennessee
- Bedford County, Virginia
- Bee County, Texas
- Belding’s ground squirrel
- Belknap County, New Hampshire
- Bell County, Kentucky
- Bell County, Texas
- Belmont County, Ohio
- Beltrami County, Minnesota
- Ben Hill County, Georgia
- Benewah County, Idaho
- Bennett County, South Dakota
- Bennington County, Vermont
- Benson County, North Dakota
- Bent County, Colorado
- Benton County, Arkansas
- Benton County, Indiana
- Benton County, Iowa
- Benton County, Minnesota
- Benton County, Mississippi
- Benton County, Missouri
- Benton County, Oregon
- Benton County, Tennessee
- Benton County, Washington
- Benzie County, Michigan
- Bergen County, New Jersey
- Beringian clade
- Berkeley County, South Carolina
- Berkeley County, West Virginia
- Berks County, Pennsylvania
- Berkshire County, Massachusetts
- Bernalillo County, New Mexico
- Berrien County, Georgia
- Berrien County, Michigan
- Bertie County, North Carolina
- Bethel, Alaska
- Bethel Census Area, Alaska
- Bexar County, Texas
- Bibb County, Alabama
- Bibb County, Georgia
- Bienville, Louisiana
- Bienville Parish, Louisiana
- Big-eared woodrat
- Big Horn County, Montana
- Big Horn County, Wyoming
- Big Stone County, Minnesota
- Billings County, North Dakota
- Bingham County, Idaho
- Black-eared deermouse
- Black-tailed prairie dog
- Black Hawk County, Iowa
- Blackford County, Indiana
- Bladen County, North Carolina
- Blaine County, Idaho
- Blaine County, Montana
- Blaine County, Nebraska
- Blaine County, Oklahoma
- Blair County, Pennsylvania
- Blanco County, Texas
- Bland County, Virginia
- Blarina
- Blarina brevicauda
- Blarina brevicauda identification
- Blarina carolinensis
- Blarina hylophaga
- Blarina peninsulae
- Blarina shermani
- Blarinini
- Bleckley County, Georgia
- Bledsoe County, Tennessee
- Blount County, Alabama
- Blount County, Tennessee
- Blue Earth County, Minnesota
- Boise County, Idaho
- Bolivar County, Mississippi
- Bollinger County, Missouri
- Bon Homme County, South Dakota
- Bond County, Illinois
- Bonner County, Idaho
- Bonneville County, Idaho
- Boone County, Arkansas
- Boone County, Illinois
- Boone County, Indiana
- Boone County, Iowa
- Boone County, Kentucky
- Boone County, Missouri
- Boone County, Nebraska
- Boone County, West Virginia