Rapid cold-hardening (RCH) is a unique form of phenotypic plasticity which confers survival advantages at low temperature. The fitness costs of RCH are generally poorly elucidated and are important to understanding the evolution of plastic physiology. This study examined whether RCH responses, induced by ecologically relevant diel temperature fluctuations, carry metabolic, survival or fecundity costs. We predicted that potential costs in RCH would be manifested as differences in metabolic rate, fecundity or survival in flies which have hardened versus those which have not, or flies which have experienced more RCH events would show greater costs than those which have experienced fewer events. One group of flies cooled to 10°C for 2h for 11 consecutive days experienced daily RCH (Hardened), whilst the other group exposed to 15°C for the same 2h period each day formed a Control group. Hardened flies had higher survival at -5°C for 2h than control flies (69±9% vs. 44±19%, P=0.04). Hardened flies showed no metabolic or fecundity costs, but had reduced average survival (P=0.0403). Thus, a major cost to repeated low temperature exposures in C. capitata is through direct mortality caused by chilling injury, although this appears not to be a direct cost of RCH.