Pentachlorophenol (PCP) was selected to assess the applicability of a toxicant criterion concentration for protecting aquatic communities in outdoor experimental channels. The criterion concentration was derived from on site laboratory bioassays. In 1982 three channels were continuously dosed with PCP for 12 wk at nominal concentrations of 48 micrograms/l (low, criterion level), 144 micrograms/l (medium, 3 X criterion) and 432 micrograms/l (high, 9 X criterion). In 1983 the low and medium treatments were repeated for 12 wk. Each year an untreated channel served as control. Invertebrate responses measured were changes in density, community composition and drift; fish responses were changes in survival, growth, reproduction and larval drift. Some ecosystem effects were detected at all treatment levels with fish being the most sensitive animals.