In 1989 a toxic Gymnodinium species was responsible for abalone (Haliotis midae) mortalities and other sub- and intertidal marine fauna in False Bay, on the south coast of South Africa. In 1995, this species was also responsible for larval mortalities in land-based abalone farms. Attempts to isolate and culture this species have revealed the presence of several Gymnodinioid species on the south coast, a number of which have been successfully isolated and cultured. The identity of these dinoflagellates have been investigated by examining the external morphology and cellular pigment composition. These studies, have to date, revealed the presence of seven different species, one of which closely resembles Gymnodinium mikimotoi. Other species identified include G. sanguineum, G. pyrenoidosum, G. pulchellum, Gyrodinium cf. corsicum and Lepidodinium viride. Another species remains unidentified. The toxicity of each culture and its filtrate was ascertained by means of an Artemia bioassay (ARTOXKIT), a routinely used method in marine and aquatic toxicology. A similar experimental procedure to the Artemia bioassay was used to investigate the toxicity of these species on both abalone larvae and spat (3mm animals). For comparative purposes similar experiments were conducted on G. mikimotoi (Isolation site: Australia), G. aureolum (Isolation site: Norway) and G. breve (Isolation site: Florida).