The leachate from a coal pile used to fuel a heating plant, drains into a small second order stream in western Massachusetts. The attendant shifts in pH from 6.8 to 4.4 and A1 from 0.03 to 3.8 ppm were not unlike those associated with acid rain. This similarity prompted a field/laboratory study to examine the effects of low pH and toxic aluminum concentrations on primary productivity of a lotic habitat. Tubular substrates, colonized by the indigenous periphyton of a clean-water tributary (control), were Transferred to the contaminated stream and monitored for structural and functional perturbations. Net productivity, measured by O2evolution, was reduced to zero after 10 days exposure to the polluted stream. Further, all diatom and cyanobacteria motility ceased. To identify the effects of pH a part from those of Al, periphyton colonizing glass coils in the laboratory were exposed to water collected from the test site and the tributary. Subsequently, the control water was acidified with 0.02 N sulfuric acid to a pH of 4.4. Oxygen evolution then declined, after 5 days, to the same levels as the laboratory communities exposed to the test waters. While low pH alone was sufficient to eliminate net productivity, it did not restrict algal movement.