The Bereket hydro-electric plant dam lakes, established on the Dalaman Stream in Dalaman town of Mugla, both meets the electricity need of the province and also irrigation need of the arable lands in the province; particularly, during summer season. This study was carried out to show environmental impact by determining the water quality and the fish fauna of the lake. Fish samples were obtained by means of electro-shocker, water analyses were performed on the field on water samples taken every month throughout a year from four stations by using ready made kits. Physico-chemical data were measured as following (minimum-maximum respectively); water temperature (13.0-29.8°C), pH (7.74-8.60), conductivity(502-837 mS cm-1), chloride (0.1-1.8 mg L-1), dissolved oxygen (4.05-9.80 mg L-1), nitrate (0.12-2.80 mg L-1), total hardness (13.5-19.0°dH) and phosphate (ALA-0.05 mg L-1). The physico-chemical data were evaluated and the water quality classes of the chosen stations in Bereket hydro-electric plant dam lakes were determined. By evaluating the physico-chemical data, according to the Water Pollution Control Regulation, the water quality classes of the chosen stations in Bereket hydro-electric plant dam lakes were determined as; 1. Class in respect to the temperature, pH, chloride, phosphate; 2. class in respect to the dissolved oxygen; acceptable in respect to the nitrate and "soft water" in respect to total hardness. At the end of the study, Capoeta capoeta bergamae, Leuciscus cephalus, Anguilla anguilla, Barbus plebejus escherichi and Cyprinus carpio species were detected in the dam lake. As a result of the ecological evaluation of the Bereket hydro-electric plant dam lake, it was found that there are no significant pollution problems.