A design for 5 X 10 m littoral enclosures that extend 10 m from the shoreline into the zone of submergent vegetation and incorporate undisturbed natural sediments for the bottom is presented and construction techniques are described. This type of enclosure was used to study the responses of caged and free-living pond biota and the physical and chemical environment to a single application of Dursban (R) (chlorpyrifos) during the summer of 1986 as part of the development of a field testing protocol for pesticides needed by the U.S. Environmental Protection Agency Office of Pesticide Programs. Chlorpyrifos was added to 12 littoral enclosures built within a mesotrophic, 2-ha pond near Duluth, Minnesota, at nominal concentrations of 0.0, 0.5, 5.0 or 20.0 micro g/L. The enclosures proved to be both economical and durable and were useful for detecting direct and indirect (ecological) effects of the pesticide. Coefficients of variation (C.V.) associated with the chemical and biological response variables typically ranged from 10 to 40%. The ability to simultaneously monitor many response variables in replicate enclosures at a relatively low cost suggests that the littoral enclosure design should be useful for studying the effects of pesticide or other pollutant additions to natural aquatic systems.