The potential of the anaerobic bacterial metabolism to raise the pH of acid lakes was tested by adding 9.1 metric tons of fresh cow manure to a 2.1 ha, 6 m deep, pH 2.9 coal mine lake near DeSoto, Illinois. This additive was expected to provide a substrate for sulfate- and iron-reducing bacteria that generate alkalinity. Lake water chemistry was monitored for 2.5 years in the treated lake and in a nearby untreated lake. The main effects of this cow manure addition were a rise in pH, iron, and sulfide and a decrease in oxygen, acidity, and aluminum in deep water of the treated lake during summer stratification. Surface water chemistry remained unchanged except or a decline in oxygen concentrations. During fall turnover, the chemistry of the entire lake returned to its pre-treatment condition. The results of this investigation indicate that anaerobic bacterial decay of organic matter can raise the pH of acid mine water. In our experiment low hydraulic retention time probably prevented a permanent increase in alkalinity and pH in the entire treated lake.