In 1999 and 2000, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program conducted a national pilot study of pesticides and degradates in drinking-water supplies, in cooperation with the U.S. Environmental Protection Agency (USEPA). William H. Harsha Lake, which provides drinking water for several thousand people in southwestern Ohio, was selected as one of the drinking-water supplies for this study. East Fork Little Miami River is the main source of water to Harsha Lake and drains a predominantly agricultural basin. Samples were collected from the East Fork Little Miami River upstream from Harsha Lake, at the drinking-water intake at Harsha Lake, at the outfall just below Harsha Lake, and from treated water at the Bob McEwen Treatment Plant. These samples were analyzed using standardized methods developed for the NAWQA Program. The purpose of this report is to provide information about pesticides and pesticide degradates in East Fork, Harsha Lake, and treated drinking water supplied by Harsha Lake. Water samples were collected from March 1999 to September 2000. The report presents a comparison of pesticide concentrations in untreated lake water and treated drinking water and a qualitative assessment of the effect of the November 1999 switchover from powdered activated carbon (PAC) to granular activated carbon (GAC) treatment on the concentration of pesticide compounds in treated lake water.