The transformation of Lambornella stegomyiae trophonts to theronts, the distribution of invasion cysts on larval Aedes albopictus cuticle, and the virulence of L. stegomyiae to Ae. albopictus and Aedes aegypti were studied in the laboratory. Transformation of trophonts into theronts was induced by a morphogenic agent released from larval Ae. albopictus homogenate. The first transformation was observed 4 hr after exposure to larval mosquito homogenate, but most transformations occurred between 12 and 16 hr. Distribution of invasion cysts on the cuticle of mosquito larvae was not uniform and most cysts were formed on the abdomen and head. L. stegomyiae was highly infective and virulent to Ae. albopictus (mortality rate: 99.53%) and Ae. aegypti (mortality rate: 90.83%) larvae.