Histologic examination of the nonnative green mussel, Perna viridis, and the native scorched mussel, Brachidontes exustus, at three locations in Tampa Bay, Florida, indicated that cycles of gametogenesis and spawning were similar between species. Major spawning periods occurred in the spring (April) in conjunction with increasing water temperature and the fall (September to November) as the water temperature was declining. Over the summer, both species appeared to undergo varying levels of redevelopment, partial spawning and resorption of oocytes. Gametogenesis was limited over the winter. Differences between populations in levels of gametogenic activity and the timing of the fall spawn were most likely related to differences in salinity and available food at the three sites. A higher fecundity (caused by the faster growth rate and greater maximum size of P. viridis) may result in displacement of native bivalves in Tampa Bay, including B. exustus.