Toxin production was measured by high pressure liquid chromatography (HPLC) when the marine dinoflagellate Protogonyaulax tamarensis (NEPCC 255) was grown under nitrogen or phosphorus limitation. The major toxins found in P. tamarensis (255) consisted of (N21-SO;-)STX (11%), (N21-SO3)NeoSTX (44%), and [(N21-SO;-)GTX2 plus (N21-SO;~)GTX3] (20%). Total toxin content on a per cell basis was high for cultures in log phase (30 to 40 fmol cell -1) and then decreased to ca 20 fmol cell -1 as the cultures entered stationary phase. There was a gradual decrease in the toxin content per cell during nitrogen-limited stationary phase to ca 3 fmol cell -~ or less. Phosphoruslimited cultures showed a markedly different response than nitrogen-limited cultures. Toxin content in P-limited cells dramatically increased at the start of stationary phase, reaching levels 3 to 4 times that observed in control and nitrogen-limited cultures. These results cannot be explained by changes in the average cell volume. Even though dramatic effects on the total toxin concentration were observed in response to nutrient limitation (N or P), the toxin composition (on a percent basis) remained constant. This suggests that the individual toxin composition of a given isolate is a fixed genetic trait and not a transient response to changing environmental factors.