The diapause response to photoperiod in Ontario populations of Cotesia melanoscela, an introduced parasitoid of the gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), was investigated under a series of stationary photoperiods and constant temperature. When the parasitic larval stages of C. melanoscela were exposed to different photoperiods, the insect exhibited a typical long-day response: long days (greater than 18 h) promoted continuous development whereas short days (less than 16 h) resulted in diapause in the cocooned prepupal stage. Within the region of the critical photoperiod (16–18 h) the diapause response was associated with larval development time. Individuals that developed more slowly were more likely to enter diapause. Cotesia melanoscela was sensitive to photoperiod throughout its entire larval period. These individual responses are used to interpret patterns of seasonality of the parasitoid in Ontario and to compare with published information from populations of C. melanoscela in New England.