The effects on water quality and aquatic life of parathion and other insecticides used in peach culture were studied. Part I describes the magnitude, origin, and duration of insecticide pollution, and Part II describes the biological effects. The study pond was located in a 40‐acre watershed where peaches are the principal crop. Parathion and minor amounts of chlorinated hydrocarbon insecticides were used for pest control. In 1960 parathion was used in the watershed from April 11 to August 1. Parathion residue (1.7 p.p.m.) was present in a surface soil sample collected before insecticide use in 1960. This indicates parathion persisted in the soil for at least nine months. Surface soil samples taken during and after parathion use in 1960 contained from 0.23 to 1.39 p.p.m. parathion. In March, prior to insecticide use in 1960, 1.90 p.p.m. parathion were recovered from pond bottom mud. Lesser concentrations of parathion (0.02 p.p.b.) were obtained from two water samples at this time. Indications are that most of the parathion present immediately before use in 1960 entered the pond adsorbed on soil during a period of accelerated erosion in March 1960. The parathion content of the pond water generally increased as the spraying season progressed. Parathion (0.01 p.p.b.) was present in water collected four months after the last application of insecticide in 1960. The parathion content of pond mud decreased during the course of the spraying season. This was believed to be due to decomposition of existing residues and a reduction in quantity of soil deposited in the pond. The fish, zooplankton, aquatic insect, and Oligochaeta populations appeared to be unaffected by parathion residues in pond water and mud. However, there was a significant reduction in immature aquatic insect numbers associated with insecticide use. The data indicate this was caused indirectly by mortality in the adult populations resulting from exposure to parathion residues in the watershed. Zooplankton were not utilized as food by bluegills when immature chironomids were relatively abundant. However, when the insect standing crop became severely reduced the fish fed almost entirely on planktonic Crustacea.