Most dissolved organic material in natural waters exist as high-molecular-weight, polymeric compounds, representing a potential source of organic matter for catabolism and growth and of essential inorganic nutrients but they are not immediately available to microorganisms because only low-molecular-weight compounds can be conveyed into microbial cells. The problem is resolved by extracellular enzymatic hydrolysis of the polymers to oligomeric and monomeric molecules. The extracellular enzymes involved are produced by heterotrophic microorganisms and microalgae. They function in periplasmic spaces, and bound to cell walls or non-living surfaces, and free in the surrounding water. Substantial extracellular-enzyme activity is associated with epilithic biofilms in streams. Evidently, the breakdown of epilithic extracellular-enzyme function, in response to toxic pollution, would adversely affect the epilithic microbiota. In the present paper we report extracellular-enzyme activity on stones from a long-term zinc-contaminated stream in Northeast England, and contrast this with activity on stones from a neighboring uncontaminated stream. The area was mined for mineral ores in the nineteenth century and abandoned zinc-rich spoil heaps are a feature of the West Allen catchment. There is much reduced abundance and diversity of invertebrates in the West Allen, where zinc concentrations were up to ten times greater than in the Eastmore » Allen. Other chemical variables, including pH and conductivity, were similar in both streams; potentially-toxic heavy metals other than zinc (Cu, Cd, Pb, Ni, Co) were not detectable. 14 refs., 1 fig., 1 tab.« less