Population densities of mature Culex spp. Larvae were reduced proportional to numbers of the planarian, Dugesia dorotocephala (Woodworth) inoculated in experimental earthen pond ecosystems; whereas the abundance of natural insect predators in these ponds was unaffected. Planaria densities also increased proportional to initial inoculation numbers. A single inoculation at the highest rate, 25 planaria/m2 of water surface, on July 15, 1975, resulted in a sustained suppression of Culex spp. below 0.25 mature larvae/400 ml-dipper sample for 51 days.