Experiments on the supercooling ability of the blowfly, Lucilia sericata (Mg.), in its prepupal diapause stage and in all life stages undergoing normal development have been carried out to determine the correlation between diapause and cold-hardiness. Results have demonstrated that the range in ability to supercool is the same in diapause larvae whether or not they have been cold-acclimated at 3 °C for varying periods of up to 9 weeks, and that diapause larvae and prepupal, non-diapause stages exhibit about the same degree of cold-tolerance. During normal development the egg and pharate adult stages have the lowest supercooling points, with the larva showing a gradual decline in ability to supercool throughout its feeding period until cessation of feeding, which occurs about 4 days before puparium formation. The results indicate that the physiological and biochemical processes leading to diapause induction do not enhance the supercooling potential of this species, and that cold-acclimation does not appear to be a prerequisite for the successful overwintering of the diapause larvae of L. sericata, at least in this part of its range in southwest British Columbia.