An experimental stream facility (ESF) has been designed to evaluate the fate and effects of consumer product chemicals as a component of municipal wastewater treatment plant (WWTP) effluent discharged to streams and rivers. The ESF is equipped with provisions for eight, 11-m experimental stream channels, computer-assisted flow control, and water quality monitoring. The stream channels were used in preliminary investigations in fall 1989 and spring 1990 to evaluate the responses of indigenous biota to the mono-alkyl quaternary cationic surfactant, lauryl trimethyl ammonium chloride (C12 TMAC). In this chapter, periphytic algal responses to C12 TMAC exposure for 8 weeks at concentrations ranging from 0 to 1250 µg/l are presented. Because consumer product chemicals are introduced into the aquatic environment as a portion of WWTP effluent, all experimental streams received 10% secondary WWTP effluent with 90% parent river water (Lower East Fork of the Little Miami River, Ohio). Structural and functional endpoints at the population and community level were employed. Endpoints varied widely in sensitivity (reflecting enhancement or impairment depending on exposure concentration) and across seasons. In general, structure and function were impaired at 250 to 1250 µg/l, concentrations well above those that would be found at 100% industry-wide use of the surfactant. Advantages of the stream mesocosm approach are discussed in the context of risk assessments for consumer product chemicals.