We examined the success of the gypsy moth larval endoparasitoid Cotesia melanoscelus among 3 host rearing methods, and 2 laboratory Lymantria dispar host strains. When reared individually, parasitism success was higher for hosts reared in 1-oz. cups than in cells of 50-cell rearing trays. Under these conditions there was no difference in C. melanoscelus success between strains. However, when parasitized gypsy moths were reared together in groups of 50, there was a distinct difference in parasitoid suitability between strains. Percent larval parasitoid emergence was 7.9x higher for Delaware strain L. dispar larvae than for NJSS larvae. Parasitoid sex ratios were not significantly affected by host strain. The reduced ability of parasitoids to successfully develop was caused almost entirely by host mortality prior to parasitoid emergence. Implications for biological control of gypsy moth by C. melanoscelus are briefly discussed. none