We examined the ecology of a Black Swamp Snake (Seminatrix pygaea) population inhabiting an isolated wetland in the upper Coastal Plain of South Carolina. The observed population structure was skewed towards mature individuals, with neonates and juveniles underrepresented, perhaps due to trapping bias. The sex ratio was biased during May and June, with females outnumbering males, but no sex ratio bias was evident at other times of the year. Seminatrix pygaea showed sexual dimorphism in body size, with females being longer and heavier than males and males having relatively longer tails than females, but there was no difference in monthly growth rates. Approximately 76.3% of the mature females captured in May and June 1998 were pregnant. Therefore, most mature females in this population probably exhibit annual reproduction, while some undergo biennial reproduction. The majority of females gave birth in July or August, with a few births occurring in September and October, and perhaps even as early as May or June. Our mark-recapture estimates suggest a population density greater than 60 adult snakes per hectare, which indicates this small aquatic snake, endemic to the Southeast, may play a large role as both predator and prey within isolated wetland ecosystems.