The response of quantitative traits in translocated populations has not been well explored. Evolutionary shifts in quantitative traits, such as life history characteristics, are of particular interest because of their obvious association with fitness. In a recent survey, Stockwell (1995) examined life history variation of four populations of western mosquitofish (Gambusia affinis) that were derived from a common ancestral population 55–58 years earlier. Three life history traits differed considerably among the four sites and appeared to be correlated with local environmental conditions. To test whether these differences were due to plasticity or rapid evolution, fish from the four sites were maintained under common-garden conditions for two generations. After two generations a genetic basis was inferred for two of the three traits (size at maturity and fat content), but not for the third trait (offspring size). Thus, rapid evolution had occurred for two life history traits in these recently established populations. This case of rapid life history evolution may be due to genetic drift and/or natural selection. These data suggest that translocations may lead to rapid evolutionary responses of targeted taxa. Rapid evolutionary responses are of special concern for ‘refuge’ populations that are maintained as a hedge against extinction.