The killing of fish fry, Tilapia zillii, by the mosquito biological control agent, Hydra americana, was studied in enamel pans, glass aquaria, in the laboratory and in outdoor ponds. It was found that the hydras readily killed the fish fry under all test conditions. The planarian mosquito predator, Dugesia dorotocephala, was found less lethal than the hydra to the fish fry at stocking rates needed for mosquito control or above these rates.