Proximate composition of small catfish, Mystus bleekeri, from Nala Daik, Sialkot, Pakistan was investigated and fluctuation in relation to body size and condition factor was carried out. Mean percentages for water, fat, protein and ash contents in the whole wet body weight of wild M. bleekeri were 77.87, 3.26, 15.01 and 3.87%, respectively. Body composition of the fish was effected by body size; however, condition factor remains constant with observed body constituents (% water, fat, protein, ash and organic contents). Highly significant (P<0.001) correlation Found between percentage water and percentage fat, protein, ash and organic contents in both wet and dry body weight. Investigation of body size on body constituents Reflected decrease in relative amount of water, skeleton (wet and dry weight), increase in fat (dry weight), protein, organic contents (wet and dry weight), and no effect on fat (wet weight) with growth of fish. Additionally, the first reference for proximate composition for M. bleekeri is provided.