Cultures of a strain ofAmphidinium carteraeHulburt isolated from a eutrophic basin in Alexandria were investigated for the optimum growth conditions, including vitamin requirements, the assimilation index and the potential toxicity of this species. A division rate of 2.7 day–1, was reached on the second and third day under optimum conditions, with and without addition of B12and biotin. The rate of C14assimilation per unit chlorophyll ranged from 0.2 to 5.4 mgC.(mg Chla)-1.h–1according to the growth conditions and to cell size. As the cell density increased in the cultures, the cell size decreased and their photosynthetic efficiency increased. Toxicity ofA. carteraeto brine shrimp resulted from grazing and not from membrane diffusion of extracellular toxins.