In the present study, I investigated the influence of temperature on the development of the entomopathogenic microsporidium Nosema lymantriae and the disease progress in the gypsy moth, Lymantria dispar (Lep., Lymantriidae). Larvae were infected with a dose of 1x10 spores and reared at 18 1C, 21 1C and 24 1C, respectively, and 16 h L/8 h D. Temperature had a significant effect on the course of infection. At higher temperatures, microsporidian developmental stages were found earlier in silk glands, fat body and Malpighian tubules. Spore release through feces of infected larvae occurred seven days earlier at 24C than at 18C as well as two days earlier than at 21C. Also the amount of released spores was significantly influenced by temperature. During the first six days of 5th instar 2.710^7 7.010^6 spores were released at 24C, 8.910^6 2.110^6 at 21C and 3.110^6 1.510^6 at 18C, respectively. While control larva consumed more and grew faster with increasing temperatures, this effect was not significant in infected larvae. Overall, infected larvae consumed less and grew slower than uninfected controls. Infected larvae reared at 24C died about 14 days earlier than those at 21 and 18C. The faster disease progress is of importance for both pathways of horizontal transmission of N. lymantriae - spores are released with feces from living larvae and from cadavers after host death. Timing of the onset of spore release has an important effect on horizontal transmission success.