Results are presented of dose-response studies of a new species of Per os and vertically transmittable microsporidian pathogen of Aedes aegypti from Thailand. The relationship between duration of exposure and percent transmission to Ae. aegypti larvae was determined. Estimated LC50s for 1st, 2nd, 3rd and 4th stage Ae. aegypti larvae were 54, 110, 210 and 1100 spores/ml, respectively. Concentrations of about 104, 5 x 104, 105 and 5 x 105 spores/ml for 24 hr were required to cause cumulative mortality of 90% or more by the time the exposed population was 14 days of age (post-hatch), when exposures were made to 1st, 2nd, 3rd and 4th stage larvae, respectively. The low IC50s indicate the pathogen is very infectious, while the LC90s suggest it is not very virulent. The fact that is might be dispersed by survivors within the target population by vertical transmission enhances its attractiveness as a potential microbial control agent. Infectivity to Ae. taeniorhynchus was demonstrated. Culex quinquefasciatus and Anopheles stephensi were not susceptible.