Musaözü Dam Lake (MDL), located on Mollaoğlu River, is the main irrigation water resource of Eskisehir, Turkey. The river and the reservoir are both under the threat of pollution primarily originated from several domestic and industrial point sources and land-based diffuse. The numerical and proportional distributions of benthic invertebrates in MDL were surveyed seasonally from January 2003 to December 2003 at six different stations. According to the results benthic invertebrate fauna consisted of Oligochaeta (42,5 %), Chironomidae larvae (30,5 %) and the varia (27 %). By evaluating the data via a Shannon-Wiener index it was found that the MDL had an index of 2,32 richness at 2nd station (inlet) and July had the widest diversity; while 4th station and January had the poorest. According to Bray-Curtis similarity index, the 5th and the 6th stations were found to be very similar to each other; while 1st and 2nd stations (outlet and inlet) were observed to be the most different from all of the other stations in terms of the dynamics of the benthic fauna. Also some physicochemical parameters and some microbiological parameters of the water were analyzed. The relationships between the dynamics of organisms and environmental parameters were supported by Pearson Correlation Index and Canonical Correspond Analysis. We found that the Dam Lake water is polluted with Cu, Cr, Ni and partly with Fe and Mn; in addition pH level of Dam Lake water is higher than inlet water. High pH and second quality levels of NO3-N, and third quality levels of NO2-N, third quality levels of Cu, Cr, Ni and, 2,32 species richness showed that similar studies should be repeated periodically in MDL so as to predict the future of the Dam Lake.