Populations of the troglobitic amphipod Crangonyx antennatus from caves in Lee Co., Virginia (U.S.A.) were investigated on both a short and long term basis. The dynamics of populations living in two distinct aquatic cave habitats (mud-bottom pools and gravel-bottom streams) were compared seasonably for one year. Sex ratios indicated a larger number of females in both pool and stream habitats. The majority of males in both habitats were found to be sexually mature throughout the year investigated. Seasonal fluctuations in female maturity were observed in both habitats, with larger numbers collected in June and August. In addition, a larger number of ovigerous females were observed in the spring, indicating the possibility of a circannian reproductive cycle in both pools and streams. The structure of populations from the caves studied appears to reflect a controlled recruitment of females from immature to mature stages. In order to determine the stability of population structure, collection data from a pool and a stream habitat for a 10-year period were analyzed. Population structures were found to be relatively stable over long periods in both habitats, with immature females comprising the dominant population class.