A flow-through laboratory microcosm using a proportional dilutor system was developed to assess effects of toxicants on algal periphyton communities. Periphyton was colonized on clay tile substrata in the field (Little Miami River, Ohio) and transferred to the laboratory for 28-day long-term exposures to copper and two anionic surfactants, dodecyl (C,2) alkyl sulfate (AS), and C,4.5 alkyl ethoxylate sulfate (AES). Five exposures of increasing test chemical concentration plus a control treatment were replicated three times and sampled weekly. Periphyton communitiesgrew well under laboratory conditions, were compositionally stable, and fairly complex. Typical community richness ranged from 16 to 22 taxa per sampled substrate. Measurement end points were chosen to assess community and population level responses that were both ecologically relevant and acceptable for interpretation by environmental regulatory authorities. Algal communities were sensitive to copper exposure and exhibited a community-level no observed effect concentration (NOEC) of 9.5 p g / L consistent with predictions by single-species studies. AS and AES exposures resulted in algal community-level NOECs of >553 and >608 pg/L, respectively, which were consistent with single-species toxicity data. Measured end points had coefficients of variation consistently below 20%, indicating their high degree of reproducibility. The test system is a viable option to classical algal single-species studies useful in determining responses of algal periphyton to xenobiotic chemical exposures. 0 1996 by John Wiley & Sons, Inc.