Samples of seven species of warmwater fish were collected above, within, and below newly impounded Saylorville Reservoir, Des Moines River, Iowa, from October 1977 to October 1978. Whole-body analyses by gas chromatography were significantly higher in river carpsucker (Carpiodes cyanazine and for the organochlorine insecticides dieldrin, p,p'-DDE, p,p'-TDE, p,p'-DDT, and heptachlor epoxide. Only the organochlorine insecticides were detected in fish tissue. Concentrations of dieldrin and heptachlor epoxide were significantly higher in river carpsucker (Carpiodes carpio) from the reservoir than in those from the river. Other species of fish showed no differences in pesticide concentration related to locality of collection.