The New Zealand isolate of the entomopathogenic fungus Tolypocladium cylindrosporum was field-tested against larvae of the crab hole breeding mosquito Aedes polynesiensis. A reduction of approximately 87% was achieved in the larval populations of all crab holes treated with T. cylindrosporum by day 21 of spore application. However, the mosquito populations recovered to the pretreatment level within two months indicating that any residual activity of this pathogen was insignificant.