The instantaneous mortality coefficient (F + M) of the commercial weakfish, Macrodon ancylodon (Bloch, 1801) caught by trawls along southern Brazilian coast (from latitude 28ºS to 34ºS), was computed by means of the annual catch variation (in number) by age per fishing effort from March 1961 to December 1964. The instantaneous mortality coefficient mean values for female and male was respectivelly 1.31 and 1.53. One year old recruits appear in the landings and the mesh selectivity operates on fish up to three years old. Three-year old class predominates in the landings.