Diarrhetic shellfish poisoning (DSP) toxins constitute a severe health risk to shellfish consumers and a serious economic risk to the shellfish industry. The most prominent producers of DSP toxins are the mixotrophic dinoflagellates Dinophysis spp., which rely on chloroplasts from their prey, the ciliate Mesodinium rubrum; i.e. they are kleptoplastidic. Studies based on field collected material have indicated that cellular toxin contents in Dinophysis spp. vary intraspecifically by several orders of magnitude, but the regulating factors of cellular toxin contents are still poorly understood. We isolated 7 strains of D. acuminata, all of which produced pectenotoxin-2 (PTX-2) in the range of 12.7 to 35.6 pg cell−1 when supplied with abundant nutrients, light and food. One strain was selected for detailed studies on the effects of irradiance and food availability on photoacclimation, photosynthesis, growth and toxin production. Photoacclimation was detected for the first time in this genus, indicating some genetic control of sequestered chloroplasts. Growth and photosynthesis increased 4-fold with irradiances from 7 to 130 µmol photons m−2 s−1 after a minimum of 2 wk under ample food conditions. Despite this, cellular PTX-2 content increased only slightly from (mean ±SE) 4.6 ± 0.3 to 7.8 ± 1.1 pg PTX-2 cell−1. Toxin production continued for up to 30 d after food was depleted, leading to a build-up of toxin in D. acuminata cells to 24.7 ± 1.7 pg PTX-2 cell−1. Thus, we demonstrate that PTX-2 content is not directly coupled to irradiance and that PTX-2 production is not directly associated with ingestion of ciliate prey. Intraspecific variations in toxicity are instead most likely governed primarily by growth phase and genetic differences between different populations of Dinophysis spp.