AbstractAldicarb was first found in ground water in Suffolk County, New York, in the late 1970s associated with its use on potatoes. Since then, there has been a substantial amount of monitoring associated with aldicarb use on potatoes nationally and on citrus in Florida. A recent monitoring study by the registrant of aldicarb supplemented limited monitoring data on other critical aldicarb uses including cotton and peanuts. This paper reports on monitoring programs and studies encompassing more than 50,000 ground water samples, of which approximately 32 percent are positive and 13 percent are above EPA's Health Advisory Level of 10 ppb. These monitoring results were used in an integrated assessment to identify areas of the United States where aldicarb is likely to leach to ground water (Lorber et al. 1989). Positive findings are reported for 61 counties in 19 states, and findings above 10 ppb are reported for 31 counties in 11 states. Monitoring data associated with the use of aldicarb on potatoes in the Northeast and upper Midwest (Wisconsin, Virginia, New York, Massachusetts, Rhode Island, Maine) are extensive and demonstrate the potential for aldicarb to impact ground water near potato‐use sites. In contrast, monitoring associated with potato use in the Northwest (Idaho, Oregon, Washington) and a three‐county potato‐growing area in Florida have shown negative results. Sampling near cotton use in the South and Southwest (Georgia, Alabama, Tennessee, Arkansa, Louisiana, Texas, Arizona) showed negative findings, while a small number of positives were found in North Carolina and South Carolina. Limited monitoring associated with peanuts showed negative results in the South (North Carolina, South Carolina, Georgia), although sampling beneath a field research site in Georgia showed trace positives. Sampling from monitoring wells established in the shallow water table aquifer beneath and downgradient from citrus use sites in Florida showed both a high frequency and high concentrations of aldicarb, with one finding above 1000 ppb and numerous findings greater than 100 ppb. However, in three separate efforts encompassing more than 1200 samples from roughly the same number of domestic drinking water wells located near citrus fields with histories of aldicarb usage, only 2 percent contained residues of aldicarb. No positives were found in 800 samples from roughly the same number of deep public drinking water wells in counties with high citrus acreage and aldicarb usage. This paper also reports on monitoring associated with lily bulb use in northern California, fernery use in Florida, sugar beet use in Montana, and smaller exploratory efforts in several other s