Fitness and rates of extinction were compared among populations of the housefly, Musca domestica L., kept either at constant effective sizes of 50, 500 or 1500 or passed through extreme founder events reducing effective size to 5. Populations were maintained for 24 generations, which for small to medium-sized mammals would be less than the 200 years suggested by Soulé et al. (1986) as necessary for maintaining viable populations of endangered species. The results demonstrate that effective population sizes have to be greater than the 50 individuals suggested by Franklin (1980) to retain fitness and escape extinction, even in the short term. In contrast to populations of constant size that exhibited monotonic decreases in fitness through time, populations established with few founders rebounded from initial inbreeding depression. However, they were less adaptable to environmental stress than constant size populations, suggesting that populations founded with few numbers may do well within a single environment but may do far less well if they are reintroduced to natural environments or exposed to rapid environmental changes.