The herbicide acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] was given conditional registration in the United States by the U.S. Environmental Protection Agency in March 1994. This registration provided a rare opportunity to investigate the occurrence of a pesticide during its first season of extensive use in the midwestern United States. Water samples collected and analyzed by the U.S. Geological Survey during 1994 documented the distribution of acetochlor in the hydrologic system; it was detected in 29% of the rain samples from four sites in Iowa, 17% of the stream samples from 51 sites across nine states, and 0% of the groundwater samples from 38 wells across eight states. Acetochlor exhibited concentration increases in rain and streams following its application to corn in the midwestern United States, with 75% of the rainwater and 35% of the stream samples having acetochlor detected during this time period. Acetochlor concentrations in rain decreased as the growing season progressed. Based on the limited data collected for this study, it is anticipated that acetochlor concentrations will have a seasonal pattern in rain and streams similar to those of other acetanilide herbicides examined. Possible explanations for the absence of acetochlor in groundwater for this study include the rapid degradation of acetochlor in the soil zone, insufficient time for this first extensive use of acetochlor to have reached the aquifers sampled, and the possible lack of acetochlor use in the recharge areas for the wells examined.