A series of 12 littoral enclosures constructed within a 2 ha, mesotrophic pond near Duluth, Minnesota were used to determine if sublethal concentrations of the insecticide chlorpyrifos (0,0,-diethyl 0-(3,5,6-trichloro-2-pyridyl) phosphorothioate) could lead to changes in the diet and growth of fathead minnow larvae. Chlorpyrifos was added to the enclosures at nominal concentrations of 0.0, 0.5, 5.0, and 20.0 microgram/L in a single application on June 16, 1986. Growth rates of larvae were significantly reduced in the treated enclosures during the 32-d study period. The most dramatic differences in the mean size of larvae from the four treatment groups were observed 15 d posttreatment. These differences corresponded to the most significant reductions in cladoceran, copepod, rotifer, and chironomid populations in the treated enclosures. These results indicate that toxicity to chlorpyrifos-sensitive invertebrate forage species forced dietary changes that led to reduced growth of native fathead minnow larvae in the treated enclosures.