• Abas wittii (Grunow) R. Ross & P.A. Sims, 1980
• Abedinium dasypus (Cachon & Cachon-Enjumet) A.R. Loeblich Jr. & A.R. Loeblich III, 1966
• Abratopdinium cardioforme S. Mao & B.A.R. Mohr, 1992 †
• Abratopdinium kerguelense S. Mao & B.A.R. Mohr, 1992 †
• Abrotanifolia barbata Stackhouse, 1809 accepted as Cystoseira barbata (Stackhouse) C. Agardh, 1820
• Abrotanifolia discors (Linnaeus) Stackhouse
• Abrotanifolia foeniculacea (Linnaeus) Stackhouse
• Abrotanifolia loeflingii Stackhouse
• Abrotanifolia mucronata (Turner) Stackhouse
• Acanthaulax acanthosphaera (Sarjeant) Sarjeant †
• Acanthaulax aceras (Eisenack) Stover & Evitt †
• Acanthaulax aculeata (Klement) Stover & Evitt †
• Acanthaulax angulosa (Gitmez) Stover & Evitt †
• Acanthaulax paliuros (Sarjeant) Sarjeant †
• Acanthaulax senta Drugg, 1978 †
• Acanthaulax spinosissima (Deflandre) Sarjeant †
• Acanthaulax venusta (Klement, 1960) Sarjeant †
• Acanthoceras azoricum (Meneghini) Kützing accepted as Ceramium azoricum Meneghini accepted as Ceramium echionotum J. Agardh, 1844
• Acanthoceras echinophorum (Meneghini) Frauenfeld
• Acanthoceras echionotum (J. Agardh) Kützing
• Acanthoceras madeburgensis Hongimann, 1910
• Acanthoceras magdeburgense Honigmann accepted as Acanthoceras zachariasii (Brun) Simonsen
• Acanthoceras oxyacanthum Kützing
• Acanthoceras shuttleworthianum Kützing, 1842 accepted as Ceramium shuttleworthianum (Kützing) Rabenhorst, 1847
• Acanthoceras transcurrens Kützing accepted as Ceramium echionotum J. Agardh, 1844
• Acanthoceras zachariasii (Brun) Simonsen
• Acanthochloris bacillifera Pascher
• Acanthochloris brevispinosa Pascher, 1930
• Acanthochloris hungarica Palik
• Acanthochloris ingens Margalef, 1956
• Acanthochloris scherffelii Pascher
• Acanthochloris verrucosa Ettl
• Acanthochrysis echinococcus Conrad, 1931
• Acanthochrysis echinocystus (Conrad) Conrad
• Acanthochrysis fusiformis Conrad
• Acanthodinium caryophyllum Kofoid
• Acanthodinium spinosum Kofoid, 1907
• Acanthodiscus antarcticus Hajós †
• Acanthodiscus clypeolus (Brun) Van Heurck †
• Acanthodiscus convexus M. Hajós & H. Stradner †
• Acanthodiscus immaculatus (Hanna) V.A. Nikolaev & J.A. Barron †
• Acanthodiscus ornatus Hajós †
• Acanthodiscus paterus (J.A. Long, D.P. Fuge & J. Smith) V.A. Nikolaev & E. Fourtanier †
• Acanthodiscus rugosus Pantocsek †
• Acanthodiscus schmidtii Jousé †
• Acanthodiscus vulcaniformis Jousé †
• Acanthogonyaulax paliuros (Sargeant) Sargeant
• Acanthogonyaulax spinifera (Murray & Whitting) H.W. Graham, 1942
• Acanthogonyaulax venusta (Klement) Sarjeant
• Acanthosphaeridium reticulatum M. Hajós & H. Stradner †
• Acanthostomella conicoides Kofoid & Campbell, 1929
• Acanthostomella elongata Kofoid & Campbell, 1929
• Acanthostomella gracilis Brandt, 1896
• Acanthostomella lata Kofoid & Campbell, 1929
• Acanthostomella minutissima Kofoid & Campbell, 1929
• Acanthostomella norvegica Daday, 1887
• Acanthostomella norvegica Kofoid & Campbell, 1929
• Acanthostomella obtusa Kofoid & Campbell, 1929
• Acaryophrya collaris Kahl, 1926
• Achilleodinium biformoides (Eisenack, 1958) Eaton, 1976 †
• Achlya ambisexualis Raper, 1939
• Achlya americana Humphrey, 1892
• Achlya apiculata de Bary, 1888
• Achlya aquatica Dayal & J. Thakur, 1969
• Achlya benekei J.S. Furtado, 1965
• Achlya bisexualis Coker & Couch, 1927
• Achlya bispora (Couch) Langsam, 1986
• Achlya brasiliensis A.I. Milanez, 1965
• Achlya caroliniana Coker, 1910
• Achlya colorata Pringsh., 1882
• Achlya conspicua Coker, 1923
• Achlya crenulata Ziegler, 1948
• Achlya debaryana Humphrey, 1893
• Achlya dubia Coker, 1923
• Achlya flagellata Coker, 1923
• Achlya glomerata Coker, 1912
• Achlya heterosexualis Whiffen, 1965
• Achlya hypogyna Coker & Pemberton, 1908
• Achlya imperfecta Coker, 1923
• Achlya intricata Beneke, 1948
• Achlya klebsiana Pieters, 1915
• Achlya lobata Ziegler & Gilpin, 1954
• Achlya megasperma Humphrey, 1892
• Achlya mucronata Ziegler, 1958
• Achlya oblongata de Bary, 1888
• Achlya oligocantha de Bary, 1888
• Achlya orion Coker & Couch, 1920
• Achlya oviparvula A.L. Rogers & Beneke, 1962
• Achlya papillosa Humphrey, 1892
• Achlya polyandra Hildebr., 1867
• Achlya proliferoides Coker, 1923
• Achlya pseudoradiosa A.L. Rogers & Beneke, 1962
• Achlya racemosa Hildebr., 1867
• Achlya radiosa Maurizio, 1899
• Achlya recurva Cornu
• Achlya sparrowii Reischer, 1949
• Achlya spinosa de Bary, 1882
• Achlya treleaseana (Humphrey) Kauffman, 1906
• Achlya turfosa Johannes, 1950
• Achnantella brevipes (Agardh) Gaillon, 1833 accepted as Achnanthes brevipes C. Agardh, 1824