A new microsporidian pathogen of Aedes aegypti from Thailand was vertically transmitted within the egg. Males played little, if any, part in vertical transmission. Infected females produced only about on-third as many progeny as uninfected females. Reduced fecundity in infected females resulted both from the production of fewer eggs and from a lower percent of hatch. In the first gonotrophic cycle not all infected females produced infected progeny, but in subsequent gonotrophic cycles some, but not all. progeny of all infected females were infected.