Cross et al. (1986) and Pfleiger and Grace (1987) reported that flathead chub (Platygobio gracilis) populations have decreased by as much as 98% over portions of their historical range. The causes of these declines are not well understood; however, degraded habitat and water quality are suspected (Lee et al. 1980). Research has indicated that portions of the Missouri River watershed in the western Dakotas are maintaining good populations of flathead chubs (e.g., Werdon 1992); however, potentially deleterious agricultural practices in these areas during the past two decades have included increases in insecticide coverage (Facemire 1993). The impacts that these chemical applications may have on healthy fish populations is a concern to agencies attempting to assess declines of native fish stocks. Carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranol N-methylcarbamate), an anticholinesterase carbamate, is commonly used as an insecticide, nematicide, and acaricide in agricultural practice around the world (Gupta 1994). Because of widespread carbofuran use, contamination has been found in the soil, air, food, water, and wildlife. The U.S. Fish and Wildlife Service has found carbofuran to be highly toxic to both birds and fishes due to damaging effects on the central nervous system (EXTOXNET 1993). However, Parkin and Shelton (1992) and Anton et al. (1993) both found that carbofuran is preferred over many other insecticides because it has a low persistence in most soil types (typically <60 days), breaks down in neutral or slightly alkaline water (half-life of 1 to 8 weeks depending upon water temperature), does not bind to sediments or suspended particles, and does not bioaccumulate. In western North Dakota, carbofuran is the most broadly applied insecticide currently being utilized by farmers and ranchers for the control of grasshoppers and other insect pests (Facemire 1993). In eastern Montana and western North Dakota, carbofuran is applied multiple times with an annual application rate of 87 g per square kilometer (USGS 1997). Fish species, such as the yellow perch (Perca flavescens) have been reported to be sensitive to carbofuran, with lethal effects on fry occurring at concentration s as low as 147 ppb; however,