Conserving aquatic resources is an important consideration in the development of environmentally sound agricultural practices. Over 1.1 million kilograms, or 2,425,500 pounds, of the insecticide terbufos are used on about 18 percent of Iowa's corn-producing acres annually, creating significant potential for contamination of surface waters. Even when properly applied, such pesticides often enter nearby streams where they can potentially affect the organisms present. Although many such agrichemical contaminants find their way to Iowa's streams and rivers, little research has been conducted to determine their effects on stream organisms.