Paralytic shellfish toxin composition in the resting cysts of the dinoflagellate Alexandrium tamarense was investigated by means of high performance liquid chromatography. A comparison was made between cysts collected from ship ballast tank sediments, natural population of motile vegetative cells collected from the area where ballast water was taken, as well as cultured vegetative cells established from the cysts and the natural plankton bloom. Total toxin concentration of the cysts (595 fmole/cell) was six-fold higher than that of the natural population of vegetative cells. They contained the same ten toxic components but in different relative abundances. The higher proportion of 11-alpha-hydroxysulfate epimers in the cysts suggests that the biosynthesis of toxins is halted at an early stage in cyst formation.