A blastoconidial preparation of the New Zealand isolate of T. cylindrosporum was tested in two field trials against larvae of the freshwater mosquito Aedes subalbirostris and the brackish water species Ae. australis. There was some reduction in the population of the Ae. subalbirostris larvae and larvae collected from the test pools exhibited signs of fungal infection when reared in the laboratory. In the brackish pools there was no observable decrease in the larval populations even though Ae. australis is the natural host of T. cylindrosporum [NZ]. Water volume fluctuations in the test pools were a major difficulty experienced when monitoring the results. Tests against field-collected non-target fauna indicated that T. cylindrosporum is non-specific in action. Species of daphnid, dixid larvae and copepods were all susceptible to infection at varying degrees.