Population characteristics of Unio crassus, an endangered freshwater mussel in many countries, are very important. In this study, we have attempted to determine the environmental conditions of Aydın-Çine Creek and the biometric properties of U.crassus. The largest length during the study period was 92.50 mm in February, and the smallest length was 37.03 mm in April. In addition, the heaviest individual was found to be 124.15 g in February, while the smallest individual was 6.69 g in April. It was found that 141 individuals were found in a group of 70.0-79.99 mm at maximum in the distribution frequency of U.crassus in Çine Creek. The water temperature was measured at 9.8-24.0ºC, and oxygen value was 6.0-10.4 mg/L during the study. In the current environmental conditions in Çine Creek, various sizes and population development of U.crassus have been identified.