Abstract: In this research, population structure of the species Aphanius danfordii found in Sirakaraağaçlar stream has been studied. During the research, sex, age, weight and length composition of 452 fishes has been examined and lenght-weight relationship and Von Bertalanffy growth function has been calculated. Female-male sex proportion has been calculated as 1.06:1.00, lenght-weight relationship and Von Bertalanffy growth function has been calculated as W = 0.0139 L3.1641, Lt = 5.149 (1-e-5.945 (t-0.502)) and Wt = 2.483 (1-e-5.945 (t-0.502)).