Reuse of treated wastewater for golf course irrigation is an increasingly popular water management option in Florida, where growth has put stress on potable water supplies. Surface water, ground water, and irrigation water were sampled at three pairs of golf courses quarterly for one year to determine if pesticides were present, and the effect of irrigation with treated effluent on ground-water quality, with an emphasis on interactions of effluent with pesticides. In addition to the six paired golf courses, which were in central Florida, ground water was sampled for pesticides and other constituents at three more golf courses in other parts of the State. This study was the first to analyze water samples from Florida golf courses for a broad range of pesticides. Statistical methods based on the percentage of data above detection limits were used to determine the effects of irrigation with reclaimed water on ground-water quality. Shallow ground water at golf courses irrigated with treated effluent has higher concentrations of chloride, lower concentrations of bicarbonate, and lower pH than ground water at golf courses irrigated with water from carbonate aquifers. There were no statistically significant differences in nutrient concentrations in ground water between paired golf courses grouped by irrigation water type at a 95 percent confidence level. The number of wells where pesticides occurred was significantly higher at the paired golf courses using ground water for irrigation than at ones using reclaimed water. However, the limited occurrences of individual pesticides in ground water make it difficult to correlate differences in irrigation- water quality with pesticide migration to the water table. At some of the golf courses, increased pesticide occurrences may be associated with higher irrigation rates, the presence of well-drained soils, and shallow depths to the surficial aquifer. Pesticides used by golf courses for turf grass maintenance were detected in ground water on seven of nine golf courses studied and in 52 percent of ground-water samples. Concentrations of pesticides in ground water at golf courses were generally low relative to gegulatory guidelines, with 45 percent of all occurrences at trace levels and 92 percent under the maximum contaminant level or guidance concentration. Two of the nine golf courses had not pesticides detectedc in ground water, and a third had only two occurrences, which were at trace levels. Theere were six occurrences of concentrations of arsenic, bentazon, or acephate in ground water above the maximum contaminant level or guidance concentration. Additionally, the following pesticides were detected in ground water from at least one site; atrazine, bromacil, diazinon, diuron, fenamiphos, metalaxyl, oxydiazon, and simazine. The fenamiphos metabolites, fenamiphos sulfoxide and fenamiphos sulfone, also were detected in ground water. Samples from wastewater treatment plants contained trace levels of atrazine, bromacil, and gamma-BHC (Lindane). Concentrations of pesticides in golf course ponds were generally low, with 60 percent of all occurrences at trace levels. All but one of the pond samples collected during the study contained at least one pesticide. The most commonly occurring pesticides in golf course ponds were: atrazine, fenamiphos and fenamiphos sulfoxide, and diuron.