• Abedinium dasypus (Cachon & Cachon-Enjumet) Loeblich Jr & Loeblich III, 1966: <0.2 mm
• Abratopdinium cardioforme S.Mao & B.A.R.Mohr †: <0.2 mm
• Abratopdinium kerguelense S.Mao & B.A.R.Mohr †: <0.2 mm
• Abruptophycus compositus Vologdin, 1962 †: <0.2 mm
• Acaciella angepena W.V.Preiss, 1972 †: <0.2 mm
• Acaciella augusta W.V.Preiss, 1972 †: <0.2 mm
• Acaciella australica (Howchin) M.R.Walter, 1972 †: <0.2 mm
• Acaciella savoryensis K.Grey & M.R.Walter, 1994 †: <0.2 mm
• Acanthina bullosa Korde, 1973 †: <0.2 mm
• Acanthina multiformis Korde, 1973 †: <0.2 mm
• Acanthocystis turfacea Chlorella virus 1: <0.2 mm
• Acanthodinium caryophyllum Kofoid, 1907: <0.2 mm
• Acanthogonyaulax acanthosphaera (Sargeant) Sargeant †: <0.2 mm
• Acanthogonyaulax paliuros (Sargeant) Sargeant †: <0.2 mm
• Acanthogonyaulax spinifera (Murray & Whitting) H.W.Graham, 1942: <0.2 mm
• Acanthostomella conicoides Kofoid & Campbell, 1929: <0.2 mm
• Acanthostomella lata Kofoid & Campbell, 1929: <0.2 mm
• Acanthostomella minutissima Kofoid & Campbell, 1929: <0.2 mm
• Acanthostomella norvegica (Daday, 1887): <0.2 mm
• Acanthostomella obtusa Kofoid & Campbell, 1929: <0.2 mm
• Acarochelopodia aduncispina Bartsch, 1977: <0.2 mm
• Acarochelopodia biunguis Bartsch, 1988: <0.2 mm
• Acarochelopodia caissara Pepato, Tiago & Rocha, 2011: <0.2 mm
• Acarochelopodia delamarei Angelier, 1954: <0.2 mm
• Acarochelopodia lapidaria Bartsch, 1991: <0.2 mm
• Acarochelopodia saruanlensis Morselli & Mari, 1986: <0.2 mm
• Acarochelopodia tarentina Morselli, Mari & Sarto, 1998: <0.2 mm
• Acaryochloris thomasi Partensky & al., 2018: <0.2 mm
• Acaryophrya collaris Kahl, 1926: <0.2 mm
• Achomosphaera alcicornu (Eisenack) R.J.Davey & G.L.Williams, 1966 †: <0.2 mm
• Achomosphaera antleriformis P.Schiøler †: <0.2 mm
• Achomosphaera argesensis E.Demetrescu †: <0.2 mm
• Achomosphaera gralliforme (Brosius) R.J.Davey & G.L.Williams, 1969 †: <0.2 mm
• Achomosphaera hirundo (Eisenack) R.J.Davey & G.L.Williams, 1969 †: <0.2 mm
• Achomosphaera improcera M.A.Islam, 1983 †: <0.2 mm
• Achomosphaera linifera (Cookson & Eisenack) Stover & Evitt, 1978 †: <0.2 mm
• Achomosphaera neptuni (Eisenack) R.J.Davey & G.L.Williams, 1966 †: <0.2 mm
• Achomosphaera recurvata K.P.Jain, S.C.D.Sah & R.Y.Singh †: <0.2 mm
• Achomosphaera sagena R.J.Davey & G.L.Williams, 1966 †: <0.2 mm
• Achomosphaera triangulata (Gerlach) R.J.Davey & G.L.Williams, 1969 †: <0.2 mm
• Achroonema articulatum Skuja, 1956: <0.2 mm
• Achroonema gotlandicum Skuja, 1956: <0.2 mm
• Achroonema inaequale Skuja, 1956: <0.2 mm
• Achroonema lentum Skuja, 1956: <0.2 mm
• Achroonema macromeres Skuja, 1956: <0.2 mm
• Achroonema proteiforme Skuja, 1956: <0.2 mm
• Achroonema simplex Skuja, 1956: <0.2 mm
• Achroonema spiroideum Skuja, 1948: <0.2 mm
• Achroonema splendens Skuja, 1956: <0.2 mm
• Achroonema sporogenum Skuja, 1956: <0.2 mm
• Acineria incurvata Dujardin, 1841: <0.2 mm
• Acineria nasuta Lepsi, 1962: <0.2 mm
• Acineta amphiasci Precht, 1935: <0.2 mm
• Acineta branchicola Precht, 1935: <0.2 mm
• Acineta compressa Claparède & Lachmann, 1859: <0.2 mm
• Acineta corophii Collin, 1912: <0.2 mm
• Acineta excirolensis Fernandez-Leborans, Yukio & Keizo, 2003: <0.2 mm
• Acineta foetida Maupas, 1881: <0.2 mm
• Acineta grahami Allgin, 1955: <0.2 mm
• Acineta harpacticicola Precht, 1935: <0.2 mm
• Acineta hokkaidensis Fernandez-Leborans, Yukio & Keizo, 2003: <0.2 mm
• Acineta laevis Dons, 1918: <0.2 mm
• Acineta laomedeae Precht, 1935: <0.2 mm
• Acineta papillifera Keppen, 1888: <0.2 mm
• Acineta pyriformis Stokes, 1891: <0.2 mm
• Acineta schulzi Kahl, 1934: <0.2 mm
• Acineta sulcata Dons, 1927: <0.2 mm
• Acineta tuberosa Ehrenberg, 1833: <0.2 mm
• Acinetides symbiotica (Daday, 1907) Curds, 1985: <0.2 mm
• Acipenserid herpesvirus 2: <0.2 mm
• Actacarus arabius Bartsch, 2004: <0.2 mm
• Actacarus australis Bartsch, 1993: <0.2 mm
• Actacarus clipeolatus Morselli & Mari, 1986: <0.2 mm
• Actacarus hastatus Bartsch, 1977: <0.2 mm
• Actacarus karoensis Abé, 1990: <0.2 mm
• Actacarus minor Bartsch, 1996: <0.2 mm
• Actacarus ponticus Bartsch, 1999: <0.2 mm
• Actacarus pygmaeus Schulz, 1937: <0.2 mm
• Actacarus spinosus Otto, 2000: <0.2 mm
• Actiniscus canadensis Bursa ex Carty, 2014: <0.2 mm
• Actiniscus elongatus Dumitrică, 1968 †: <0.2 mm
• Actiniscus flosculus S.Locker & E.Martini, 1986 †: <0.2 mm
• Actiniscus laciniatus S.Locker & E.Martini, 1986 †: <0.2 mm
• Actiniscus pentasterias (Ehrenberg) Ehrenberg, 1844: <0.2 mm
• Actiniscus squamosus S.Locker & E.Martini, 1986 †: <0.2 mm
• Actinocyathula crenata (Fraipont, 1878) Curds, 1987: <0.2 mm
• Actinocyathula homari (Sand, 1899) Curds, 1987: <0.2 mm
• Actinodinium apsteinii Chatton & Hovasse: <0.2 mm
• Actinophycus kordeae Vologdin, 1962 †: <0.2 mm
• Actinophycus obrutschevii Korde, 1954 †: <0.2 mm
• Actinophycus parvus Vologdin, 1962 †: <0.2 mm
• Actinophycus ramosus Vologdin, 1962 †: <0.2 mm
• Acuasiphonoria ordovica Lijing Liu, Yasheng Wu, H.J.Yang & Riding, 2016 †: <0.2 mm
• Adenoides eludens (Herdman) Balech, 1956: <0.2 mm
• Adenoides sinensis H.Gu, X.Li & Z.Luo, 2018: <0.2 mm
• Adinimonas oviformis J.Schiller, 1928: <0.2 mm
• Adriamonas peritocrescens Verhagen, Zölffel, Brugerolle & D.J.Patterson, 1994: <0.2 mm
• Adrianema adriaticum (Ercegovic) G.De Toni, 1936: <0.2 mm
• Aegeococcus anagnostidisii Konstantinou & Gkelis, 2020: <0.2 mm
• Aegeococcus thuretii Konstantinou & Gkelis, 2020: <0.2 mm