Prymnesium parvum causes harmful algal blooms worldwide that are often associated with massive fish-kills and subsequent economic losses. Most of our knowledge of the toxicity of P. parvum derives from bioassays since methods for the identification and quantification of their toxins have been lacking. Recently, a quantitation method was developed for the causative lytic toxins, the prymnesins. Here, we for the first time present data on the influence of irradiance on cellular content and production of prymnesins under nutrient replete conditions in two P. parvum strains, which both produce B-type prymnesins. Large differences were observed between the two strains with regard to the influence of irradiance on prymnesin cell quota and production rates. At the highest irradiance level (550 µmol photons m-2 s-1), the cellular prymnesin quota was thirty times higher in strain K-0081 strain than in K-0374. The cellular prymnesin quota and production rates were closely linked to rates of growth and photosynthesis in strain K-0081, while this was not the case for K-0374. Yet, growth rate did explain the differences in prymnesin quota in the two strains. Consequently, the maximum prymnesin production rate (414 attomol cell-1 d-1) was only about three times higher in strain K-0081 than in K-0374, and revealed an optimum at the same irradiance of 200 µmol photons m-2 s-1 in both strains. At low irradiance levels, the difference in production rates between both strains became smaller, with 41 and 49 attomol cell-1 d-1 for K-0081 and K-0374, respectively. The carbon content of prymnesins made up for ~3% and <1% of the total cellular carbon content in strains K-0081 and K-0374, respectively. The fraction of extracellular dissolved prymnesins was measured for strain K-0081, where it accounted for 14-30% of total prymnesin concentration in the cultures, irrespective of irradiance. The concentrations of prymnesins released to the water by the K-0081 strain were not significantly influenced by irradiance. Overall, we observed comparable responses in growth and photosynthesis of both tested strains toward changes in irradiance. However, the effects of irradiance on prymnesin quota and production rates were remarkably different between the two strains.