Freeze tests were conducted throughout the winters of 1987–1988 and 1989–1990 to determine the comparative supercooling points ofUrophora affinisFrauenfeld andU. quadrifasciata(Meigen) larvae, and to assess the effects of site and midwinter temperature change on the supercooling ability ofU. affinis. The supercooling capacity ofU. affiniswas significantly greater than that ofU. quadrifasciata; the mean supercooling point forU. affinisduring the two winters was −35.1°C compared with −31.0°C forU. quadrifasciata. Regression analyses showed a significant, negative quadratic relationship between supercooling point and collection date during the winters of 1987–1988 and 1989–1990 for both species. Differences in supercooling ability amongU. affinislarvae collected from six different Bitterroot Valley sites were infrequent, of small magnitude, and inconsistent over the 2 yr sampled. The supercooling ability ofU. affinismaintained at +25°C was significantly less than that ofU. affinisin the field but the supercooling ability ofU. affinismaintained at or slightly below freezing was generally comparable with that ofU. affinisin the field. Based upon this study, both fly species can be expected to survive the winter in most areas of western Montana and the Pacific Northwest, but onlyU. affinisshould be considered for release in eastern Montana and other regions east of the Continental Divide where winter temperatures are often lower than −35°C.