The coupling of toxicity expression with cell-cycle phases was studied in the toxic marine prymnesiophyte Chrysochromulina polylepis Manton & Parke, Clone B1511. Cell synchronisation of cultures in exponential or early stationary growth phases under nutrient-replete conditions was achieved by manipulation of the photoperiod. Chlorophyll a (chl a) and cell number increased in a stepwise manner, but were asynchronous, with chl a increasing during the light period and cell number increasing during the dark period. In the course of the light period, nearly all cells clustered in the G1 (Gap 1) phase, which lasted for about 20 h. DNA synthesis (S phase) occurred mainly in the dark during a discrete period (about 4 h) and G2 (Gap 2) and mitosis (M) were always completed before the end of the dark period. Toxicity expression, measured by the erythrocyte lysis assay (ELA), exhibited a dramatic drop in LC50 values (increase in toxicity) during the light period, although this effect was less pronounced after the first 2 generations of cell division when the cultures had entered the stationary phase. Similarly, haemolytic activity per unit cell volume decreased by a factor of 3 to 4 during the dark period over the first 48 h, but became irregular towards the end of the experiment. In this study, the light-dependent effect on toxicity and relationship to discrete phases of the cell cycle are demonstrated for the first time in a prymnesiophyte. © Inter-Research 2005.