A
- Humayun Abdulali (1914–2001), Indian ornithologist
- Aziz Ab'Saber (1924-2012), Brazilian geographer, geologist and ecologist
- Erik Acharius (1757–1819), Swedish botanist
- Johann Friedrich Adam (18th century – 1806), Russian botanist
- Arthur Adams (1820–1878), English physician and naturalist
- Henry Adams (1813–1877), English naturalist and conchologist
- William Adamson (1731–1793), Scottish botanist (abbr. in botany: Aiton)
- Michel Adanson (1727–1806), French naturalist (abbr. in botany: Adans.)
- Edgar Douglas Adrian (1889–1977), British electrophysiologist, winner of the 1932 Nobel Prize in Physiology or Medicine for his research on neurons
- Adam Afzelius (1750–1837), Swedish botanist
- Carl Adolph Agardh (1785–1859), Swedish botanist
- Jacob Georg Agardh (1813–1901), Swedish botanist
- Louis Agassiz (1807–1873), Swiss zoologist
- Alexander Agassiz (1835–1910), American zoologist, son of Louis Agassiz
- Nikolaus Ager (1568–1634), French botanist
- Pedro Alberch i Vié (1954–1998), Spanish naturalist
- Bruce Alberts (born 1938), American biochemist, former President of the United States National Academy of Sciences
- Boyd Alexander (1873–1910), English ornithologist
- Horace Alexander (1889–1989), English ornithologist
- Richard D. Alexander (born 1930), American evolutionary biologist
- Wilfred Backhouse Alexander (1885–1965), English ornithologist
- Alfred William Alcock (1859–1933), British naturalist
- Salim Ali (1896–1987), Indian ornithologist
- Frédéric-Louis Allamand (1736 – after 1803), Swiss botanist (abbr. in botany: F.Allam.)
- Warder Clyde Allee (1885–1955), American zoologist and ecologist, identified the Allee effect
- Joel Asaph Allen (1838–1921), birds, mammals
- George James Allman (1812–1898), British naturalist
- Prospero Alpini (1553–1617), Italian botanist
- Sidney Altman (born 1939), Canadian-born molecular biologist, winner of the 1989 Nobel Prize in Chemistry for his work on RNA
- Bruce Ames (born 1928), American biochemist, inventor of the Ames test
- José Alberto de Oliveira Anchieta (1832–1897), Portuguese naturalist
- George French Angas (1822–1886), English explorer, naturalist, conchologist and painter
- Jakob Johan Adolf Appellöf (1857–1921), Swedish marine zoologist
- Aristotle (384 BC-322 BC), Greek philosopher
- Peter Artedi (1705–1735), Swedish naturalist
- Gilbert Ashwell (born 1916), American biochemist, pioneer in the study of cell receptor
- Jean Baptiste Audebert (1759–1800), French naturalist
- Jean Victoire Audouin (1797–1841), French zoologist
- John James Audubon (1786–1851), American ornithologist
- Charlotte Auerbach (1899–1994), German geneticist, founded the discipline of mutagenesis
- Richard Axel (born 1946), Nobel prize winning physiologist
- Julius Axelrod (1912–2004), American biochemist, winner of the 1970 Nobel Prize in Physiology or Medicine for his research on catecholamine neurotransmitters
- William Orville Ayres (1817–1887), American physician and ichthyologist
- Félix de Azara (1746–1811), Spanish naturalist
Read more about this topic: List Of Biologists