An overview of a study to evaluate a site-specific water quality criteria for pentachlorophenol (PCP) in outdoor experimental streams is presented. The site-specific criterion was calculated from acute toxicity results for eight resident species and the relationship between acute and chronic toxicity of PCP. The PCP concentration expected to protect aquatic life (30-day average criterion concentration) was less than or equal to 48 micrograms/l. Outdoor experimental streams were subsequently dosed continuously for 84 days at 48, 144, and 432 micrograms PCP/l. Measurements of the biological structure and ecosystem processes within the exposure streams were compared to a control system. Effects on snails were found only at 432 micrograms/l, but effects on fish, periphyton, and system metabolism were found at 432, 144, and 48 micrograms/l. The small differences between the criterion-dosed stream (48 micrograms/l) and the control stream may have been caused by PCP or interstream variation.