Vertical transmission and the overwintering success of three different microsporidia infecting Lymantria dispar (Lepidoptera: Lymantriidae) larvae were investigated. Endoreticulatus schubergi, a midgut pathogen, was transmitted to offspring via female and male via the egg chorion (transovum transmission). Between 8% and 29% of the emerging larvae became infected. No spores of E. schubergi were found in surface-washed eggs. Nosema lymantriae, a microsporidium that causes systemic infections, was transovarially transmitted. Between 35% and 72% of the progeny were infected. Vairimorpha disparis, a fat body pathogen, was not vertically transmitted. The infectivity of spores that overwintered in cadavers of infected L. dispar varied by species, placement in the environment, and weather conditions. Spores of E. schubergi were still infective after an eight month exposure period of cadavers on the ground. Spores of N. lymantriae and V. disparis remained highly infective only when cadavers overwintered under a more or less continuous snow cover for four months.