Three geographical strains of the blow fly, Calliphora vicina, were tested for cold tolerance at 0°, - 4°and - 8°C. Survival to eclosion after 1 to 18 days of cold exposure was greater for diapause-destined larvae than for nondiapause-destined larvae of the two northern strains (Nallikari, Finland 65°N and Edinburgh, Scotland 55°N) but not for the southernmost strain (Barga, Italy 44°N) where no clear differences were apparent. Diapause-destined larvae of the Edinburgh strain were more cold tolerant than those from Nallikari, at both - 4°and - 8°C, a difference possibly attributable to the long-lasting snow cover in the more northern locality, which might insulate the overwintering soil microclimate. At 0°C, however, Nallikari larvae were more cold tolerant than Edinburgh or Barga. This was also the case for nondiapause-destined larvae, indicating the cold tolerance may occur, in part, independently of the diapause programme. In all three strains diapausing larvae were more cold tolerant than same-age (nondiapausing) pupae. For Nallikari, but not Barga, wandering larvae from short-day exposed flies, therefore initially programmed for diapause, but diverted from the diapause pathway by larval breeding at 19°C, were significantly more cold tolerant than nondiapause larvae from long-day parents, indicating some maternal regulation of larval cold tolerance. There was, however, no evidence for an additional cold hardiness in larvae acclimatised to cold by a gradual reduction of temperature.