The parameters of Bertalanffy’s growth equation for Patagonian rockcod (Patagonotothen guntheri shagensis) were calculated by using age and length-weight data of research and fishery survey cruises in the Shag Rocks area from 1978 to 1986. The natural mortality rate was assessed by six different methods and was within a range of 0.63 to 1.06. The mean value of natural mortality M=0.8 was used in calculations and the optimal age and length for harvesting Patagonian rockcod were found to be 2.5 years and 12.6 cm.