Abstract Surface water taken from 12 different areas in the period from July 2002 to June 2003 from Sapanca Lake, (Sakarya, Turkey) was analyzed in order to detect levels of bacterial contamination, bacterial metabolic activity, and chemical content. Level of bacteria was examined with respect to Fecal coliform, Escherichia coli and Salmonella spp. The contribution of capsule-bearing bacteria to the total number of bacterioplankton community was enumerated in order to assume the metabolically active number of bacteria. Levels of dissolved oxygen, nitrate, nitrite, orthophosphate, total hardness and calcium were calculated. Bacterial counts were made using Membrane Filtration Technique. Capsulated and non-capsulated bacteria were determined using a modified negative staining technique. As a result, the highest level of F. coliform found was 24x103 MPN/ 100 ml in the surface water samples, which were taken from the western side of the Lake. While Salmonella spp. was found to be positive, contribution of capsulated bacteria to the total bacteria was found to be higher (19.6 %) in the same region. Results of this study have shown that level of indicator bacteria and percentage of potentially active bacteria are higher in samples of surface water collected from western part of the Sapanca Lake. Evaluation of the relations between surface fecal coliform and other environmental factors showed that the western region of the Lake was exposed to sewage wastes.