The centric diatom, Thalassiosira weissflogii Grun., can be induced to undergo spermatogenesis by exposing cells maintained at saturating levels of continuous light to either dim light or darkness. Using flow cytometry to determine the relative DNA and chlorophyll content per cell, the number of cells within a population that responded to an induction signal was measured. From 0 to over 90% of a population differentiated into male gametes depending upon both the induction trigger and the population examined, regardless of the average cell size of the population. Through the use of synchronized cultures, we demonstrated that responsiveness to an induction trigger was a function of cell cycle stage; cells in early G1 were not yet committed to complete mitosis and were induced to form male gametes, whereas cells further along in their cell cycle were unresponsive to these same cues. A simple model combining the influence of light on the mitotic cell cycle and on the induction of spermatogenesis is proposed to explain the observed diversity in population responses to changes in light conditions.