The aim of this study is to provide necessary information on the biology of mosquitofish in Seyhan Dam Lake in Adana, which is located in southern part of Turkey. The population structure of eastern mosquitofish Gambusia holbrooki Girard, 1859 was studied in Seyhan Dam Lake. A total of 1582 specimens (772 males and 810 females) were collected monthly from January to December, 2007. The age composition of the sample range between 0+ and 2+in both sexes. The sex ratio was M:F= 1:1.04. The length-weight relationship calculated for all individuals, with W=0.0129 x L-2.927. von Bertalanffy growth parameters were for males, L-infinity=3.31cm, K=0.6597 year(-1), t(0) =0.9483 year; for females L-infinity=6.62cm, K=0.2369 year(-1) t(0) =0.9259 year; for all samples, L-infinity=5.84cm, K=0.2369 year(-1), to =-1.0740 year. A total of 102 stomachs were examined and total of organisms identified from alimentary canal. The diet mainly consist of Diptera (Chironomidae (pupa) 29.40%; Diptera (adult) 24.50%), Egg (mosquito 14.70%; different organisms 1.96%), Crustacea (Copepoda 9.80%, Cladocera 3.92%), Coleoptera 4.90%, Hemiptera 1.96%, Fishes (G. holbrooki 3.92%), Other Hymenoptera 1.96%, Tricoptera 0.98%, Plecoptera 0.98%, Formicidae 0.98%. These data were conducted to compare with the results of other studies to carry out for othei geographic areas.