Amblygaster sirm is the dominant fish species in the purse seine fishery of the south­ west coast of Sri Lanka. Stock Assessment studies of A^_ sirm were conducted, based on length -frequency data collected from February 1985 to December 1987. Growth parameters (L^ and K) of the von Bertalanffy equation and mortality coefficients (Z, M and F) of the exponential decay model were calculated for each of the three years. These parameter estimates were utilized in relative yield-per- recruit analysis to evaluate the present level of exploitation. Relatively high growth rates (mean K = 2.04 and mean 1,*,= 23.45 cm) were obtained. Natural mortality was estimated at 3.07. These estimates inply fast turnover rates allowing fishing pressure to be maintained at relatively high levels. The maximum relative yield/recruit could be obtained at a high exploitation rate of 0.8. The present exploitatio -n rate remains at 0.47. It is suggested that the present fishing effort should be maintained although an increase in effort would result in a slight increase in the relative yield.