Isolates of the centric diatom, Thalassiosira weissflogii Grun., were maintained in exponential growth under constant, favorable conditions for nearly 2 years. During this interval, each culture underwent periodic increases and decreases in mean cell size, a behavior predicted for diatom populations alternating between sexual and asexual reproduction, respectively. The overall patterns of cell size change displayed by each culture, however, were unique. The maximum size of newly enlarged cells varied among isolates and within a given isolate over time. Consequently, both the timing and rate of increase in mean cell size also varied despite the fact that the minimum average cell size obtained by the various cultures was relatively constant. The most consistent feature among the isolates was the rate of decrease in mean cell size, a value determined by the physical constraints of the diatom frustule during mitotic divisions. We hypothesize that the extent of the variability exhibited by these cultures results from the fact that an inherent feature of diatom populations is a constantly changing genetic composition.