The predatory effectiveness of the planarian, Dugesia dorotocephala, against populations of chironomid midges and mosquitoes in earthen experimental ponds was studied during the summers of 1975, 1976 and 1977. In 1975, inoculation levels of 10 and 25 planaria/m2 of pond surface area produced reductions of 28.5% and 52.3%, respectively, in populations of Dicrotendipes sp., Tanytarsus spp., Procladius spp. and Cricotopus spp. midges. In 1976, the midge larvae were reduced by 36.5%, 51.0% and 58.3% at levels of 25, 50 and 100 planaria/m2, respectively. In 1977, levels of 25 and 50 planaria/m2 produced 37.8% and 47.5% midge control, respectively. Larval populations of Culex mosquitoes were reduced in 1976 by 71.8%, 75.2% and 77.0% at inoculations of 25, 50 and 100 planaria/m2, respectively. In 1977, Culex larvae were reduced 63% with 25 planaria/m2 and 73% with 50 planaria/m2 . The planaria in inoculated ponds persisted throughout each evaluation period, and showed slight population increases, but these increases were inconsistent and non-proportional to their initial inoculation densities.