Abstract.Eretmocerus eremicus is a parasitoid wasp that is not native to Britain. It is a biological control agent of glasshouse whitefly and has recently been released under licence in Britain for the first time. This study assessed the effect of low temperature on the outdoor establishment potential of E. eremicus in Britain. The developmental threshold calculated by three linear methods was between 6.1° and 11.6 °C, with a degree-day requirement per generation between 256.3 and 366.8° day−1. The supercooling points of non-acclimated and acclimated larvae were similar (approximately −25 °C). Non-acclimated and acclimated larvae were subject to considerable pre-freeze mortality, with lethal temperature (LTemp50) values of −16.3 and −21.3 °C, respectively. Lethal time experiments indicated a similar lack of cold tolerance with 50% mortality of both non-acclimated and acclimated larvae after 7 days at −5 °C, 10 days at 0 °C and 13 days at 5 °C. Field trials showed that neither non-acclimated nor acclimated larvae survived longer than 1 month when exposed to naturally fluctuating winter temperatures. These results suggest that releasing E. eremicus into British greenhouses would pose minimal risk because typical British winter temperatures would be an effective barrier against establishment in the wild.