Abstract The production of diapause larvae in two strains of the parasitic wasp, Nasonia vitripennis, has been investigated. This species is a ‘long-day insect’ with a temperature-modified photoperiodic response. A facultative diapause occurs at the end of larval development, but it is the adult female which is sensitive to photoperiod and temperature. Females exposed to short daylength and low temperature produce the majority of their offspring as diapause larvae and females exposed to long daylength and high temperature produce few diapause larvae. Once the egg has been deposited the course of its development cannot be modified by changes in photoperiod and temperature applied to the immature stages. The critical daylength in the C strain (Cambridge, England) is close to 15 to 15 1 4 hr per day, and that in the WH+ strain (Woods Hole, Massachusetts) about 13 1 2 hr per day.