ABSTRACT.1Once pupal diapause had been terminated, over‐wintering cabbage root fly (Delia radicum(L.)) pupae from Wellesbourne required a further 14 days at 20°C for most of the flies to emerge.2There were considerable variations in the rates of fly emergence from thirteen populations of cabbage root fly pupae collected between latitudes 50° 42′ and 54° 59′ in England and Wales. These thirteen populations could be grouped into early‐, intermediate‐ and late‐emerging types. In the early‐emerging type, flies emerged within 14 days at 20°C whereas in the late‐emerging type emergence was protracted and was completed only after 100 days at 20°C in one population from Halsall, Lancashire. In the intermediateemerging type, approximately two‐thirds of the flies emerged within 14 days at 20° C, the remainder taking considerably longer.3The intermediate‐emerging types could be just mixtures of the early‐ and late‐emerging types.4Subjecting pupae to diapause‐breaking temperatures (4°C) for up to 1 year failed to shorten the time to subsequent fly emergence in any of the populations.5Populations of early, intermediate‐ and late‐emerging fies could be selected from a parental population, heterogeneous with respect to emergence, within one generation.6The type of emergence that occurred in a locality was not correlated with latitude.7Any models developed for forecasting the most appropriate time to apply insecticide in a locality will have to include information about the emergence pattern of