Rates of cell volume increase in individuals of five genera of centric marine diatoms were measured using time-lapse video microscopy. In continuous light, size increased continuously in and , while steps were observed during the growth of and sp. In the latter three species, the duration of the periods of no growth were well correlated with the generation time for individual cells. When the species exhibiting continuous growth in constant light were grown in a light/dark cycle of 12:12 h, the rate of size increase during the dark period was on average slower than during the light. Behaviour of individual cells was highly variable, however, and in appeared to be related to the previous light history of each cell. The results suggest a regulatory coupling between the cell cycle and the time evolution of cell volume.