Pigment analyses by TLC were carried out during and after the peak of a Peridinium bloom in Lake Kinneret (Israel). During the experimental period considerable changes occurred in the absolute and relative amounts of the various pigments, especially /3 carotene, diadinoxanthine, peridinin, chlorophyll a and chlorophyl c. The ratio between total carotenoid to total chlorophyll content is also followed. Our findings show that considerable changes in pigment composition in natural phytoplankton can occur while the species composition remains constant.