In Manitoba, first-stage larvae of Microctonus vittatae Mues. overwinter in diapausing adults of two rape-infesting chrysomelids, Phyllotreta cruciferae (Goeze) and Phyllotreta striolata (F.). The parasite larvae resume their development in May after the hosts have finished overwintering, and mature parasite larvae emerge and spin cocoons from which adults emerge late in May and during June. Usually there are four parasite generations each year; most of the larvae in the first three generations are non-diapausing, whereas most first-stage larvae of the fourth generation develop briefly and then enter diapause in September or October and overwinter. Early attempts to propagate M. vittatae for laboratory studies were unsuccessful because most of the immature parasites either died or entered diapause. Preliminary tests suggested that, as in some other parasite species (Saunders 1965; McNeil and Rabb 1973; Anderson and Kaya 1974), the photoperiod at which the parasites were propagated influenced the incidence of diapause. The following three experiments examined this hypothesis and attempted to identify rearing conditions that would prevent diapause or limit its incidence in M. vittatae.