The goal of this research was to understand what factors determine whether a diatom cell can exit the mitotic cell cycle to undergo gametogenesis and subsequent auxospore formation. Using flow cytometry, I found that the centric diatom, Thalassiosira weissflogii, could be induced to undergo spermatogenesis by exposing cells maintained in continuous light to either dim light or darkness. The use of populations representing distinct cell cycle distributions indicated that cells in early G1 could be triggered to form male gametes. Whereas cells further along in their cell cycle were unresponsive to these same cues. The size distributions of numerous isolates maintained under identical conditions were examined over time to determine the relation between cell size and the ability to undergo sexual reproduction. Each isolate underwent periodic increases and decreases in mean cell size as expected for populations alternating between asexual and sexual reproduction. However, the timing of the onset of sexual reproduction, the rate of increase in mean cell size, and the size of the post-auxospores created during a sexual event varied among isolates and within a given isolate over time. This variable behavior was hypothesized to occur because the genetic composition of diatom populations varies over time. Keywords: Medicine.