Population densities and factors affecting the distribution of the troglobitic (i.e., obligatory cavernicole) amphipod Crangonyx antennatus were examined in two distinct aquatic habitats. Observations were made seasonally for one year in six Lee Co., Virginia (U.S.A.) caves, three with mud-bottom pools and three with gravel-bottom streams. Pool habitats were found to contain greater C. antennatus densities than stream habitats, while the highest seasonal densities were recorded in August. Availability of food is considered to be the major factor allowing these increased population densities. The population distribution of C. antennatus was not found to be related to current velocity, water depth, macro-detrital food sources or isopods densities in both pool and stream habitats. The distribution of C. antennatus was observed to be related to substrate type. Substrate selection appeared to be passive in five of the amphipod populations, while in one cave, substrate selection may be influenced by competitive interactions. The fact that population distributions were not related to environmental parameters which differ markedly between habitats indicates that this troglobitic species retains ecological flexibility.