The objective of the present study is to report for the first time the occurrence of Spermatozopsis similis H.R. Preisig et M. Melkonian in a marine environment, describing the morphology and its abundance in Ría del Jabalí;Bahía Anegada, Buenos Aires, Argentina. Samples were taken monthly between June and November 2006 and wereobserved under light and scanning electron microscope. Specimens of S. similis showed biflagellate, fusiform andslightly spiraled cells, with flagella of unequal length. S. similis was an important component of phytoplankton in Ríadel Jabalí between June and August 2006, reaching its maximum abundance in August (6 770 cél.ml-1), during thisperiod there were observed very high and fluctuating values of salinity. This paper is the first record of S. similis ina marine environment.