A preliminary assessment was made of the microbial control potential of a Helicosporidium sp. (Protozoa: Helicosporida) from Aedes aegypti and Culex quinquefasciatus in Thailand. Aedes aegypti was used as an experimental host in which to determine dose-mortality efficiency of the pathogen. Its storage properties and its susceptibility to de gradation by various stressors likely to be encountered in the aquatic environment, such as heat, pH, salinity, detergents, and ultraviolet light were analyzed. The infectivity of the helicosporidian spore to the fall armyworm, Spodoptera frugiperda, by injection and reduced infectivity of the resulting spores was demonstrated. The infectivity of the pathogen for Anopheles dirus, An. maculatus, Ae. taeniorhynchus, and Toxorhynchites splendens was quantified. It was concluded that the agent was unlikely to be useful as a practical, cost-effective microbial control agent until various technological advances are made.