One of the toxic dinoflagellates Protogonyaulax tamarensis isolated in Ofunato Bay was grown in axenic state to clarify the effects of coexistent bacteria on the toxin productivety and toxin profile of the organism. The toxicity per one cell changed according to the culture period, indicating that the variability of toxicity is associated with the physiological condition of the dinoflagellate itself, but not with the bacteria. The toxin yields in axenic culture were much higher than in unialgal culture, probably as the result of the slower growth in the former condition. A toxicity as high as 170×10^-6 MU/cell was observed at the earliest stage of growth; this was comparable to those of the wild motile cells and cysts of the same species. The toxin profile of the cells cultured in bacteria-free condition was essentially the same as that of unialgally cultured ones.