The present study investigates the effect of brackish (7 PSU) and marine (26 PSU) salinity on physiological parameters and intra- and extracellular toxicity in 4 strains of Prymnesium parvum Carter. The different P. parvum strains were grown in batch cultures in 2 trials under different experimental conditions to test the development of intra- and extracellular toxicity during growth. The response of P. parvum toxicity to salinity was validated using 2 protocols. Intra-specific variations in growth rate, maximal cell density (yield) and cell morphology were controlled by salinity. Extracellular toxicity was higher at 7 PSU in all strains, but no correlation was found between intra- and extracellular toxicity. The variation of extracellular toxicity in response to salinity was much greater than that of intracellular toxicity, which indicates that P. parvum may be producing a variety of substances contributing to its various types of 'toxicity'.