One hundred sixty-eight specimens ofPotamilus alatus(Say, 1817) were collected approximately monthly from the Wheeler Reservoir (Tennessee River mile 298), Alabama, USA, between February 1995 and March 1996. Microscopic and gross examinations of gonadal tissue and marsupia were used to determine seasonal timing of gamete production, spawning, brooding and glochidia discharge. All individuals examined were dioecious. Spermatogenesis occurred between July and August. Spermatozoa numbers peaked in late August and male spawning occurred over a 2–3-week period in late August and early September. Oogenesis and female spawning had patterns similar to spermatogenesis and male spawning. Brooding of glochidia occurred between September and April. Glochidia discharge occurred in April and possibly early May, and coincided with the onset of the spawning season of the reported fish host,Aplodinotus grunniens(Rafinesque, 1819). The temporal patterns of gamete production and spawning inP. alatusof the subfamily Lampsilinae are in sharp contrast to members of the subfamily Ambleminae that have been similarly studied. This suggests that detailed studies of gamete production and spawning may be helpful in understanding unionid phylogeny and character evolution.