Abstract: From May 1986 to December 1987, 928 specimens of Mystus macropterus (Bleeker) were collected from the middle and lower reaches of the Jialing River for age determination and growth study. 826 specimens were aged based on the cross-section of decalcified pectoral spine. Age composition of the catch was analysed. The regression of body length on the "redius" of the spine section was calculated, and optimal equations for growth were selected. Backcalculations from the radii of annuli on the spine seetion were carried out. Based on the theoretical value of body length and body weight, growth indexes were calculated. Growth difference in body weight between male and female was demonstrated. The diffefence is related to the fact that the male mature one year earlier than the female. The paper also discussed the cause and biochemical mechanism for annual ring formation. It is confirmed that use of the cross—section of the decalcified spine for the study of age and growth of small—size Bagridae is reliable and easy to perform.