Hawaiian tree snails in subfamily Achatinellinae have experienced drastic population declines and range reductions as a result of habitat destruction, overcollection, and introduced predators. In 1991, 11 individuals from one of the last remaining populations of Achatinella fuscobasis, a federally listed species, were brought to the University of Hawai‘i's tree-snail captive-rearing facility. After increasing in population size to 441 snails in 2005, the population subsequently declined to only 120 snails by 2014, a 73% reduction. In this study, we assessed demographic and genetic changes that occurred in the captive-bred population of A. fuscobasis. Demographic data were compared to genetic data collected from 11 microsatellite markers for the F1 to F3 generations to investigate the role bottleneck effects such as loss of genetic diversity and inbreeding may have had on the population decline. Demographic measures such as juvenile survival and survival to maturity remained constant for the F1 and F2 generations, and significantly decreased in the F3 generation. There was no difference in genetic diversity measures such as allelic richness, gene diversity, and observed heterozygosity among generations, but all generations were characterized by low genetic diversity and high inbreeding. The drastic population decline appears to have been due to stochastic demographic dynamics characteristic of small populations, with bottleneck effects that occurred before the founding of the ex situ population acting in synergy with factors such as disease.