Trypanosoma congolense releases a number of hemolytic free fatty acids on autolysis of which the most potent is linoleic acid. These fatty acids can lyse washed rat and bovine erythrocytes in vitro. Autolysed T. congoleuse will cause increased fragility of erythrocytes in whole rat blood but not in whole bovine blood. The lack of effect of these fatty acids on bovine erythrocytes tends to be confirmed by the failure of this material to influence the survival of these cells in vivo. Calculations based on the amount of free fatty acids required to cause hemolysis in vivo also confirm that this mechanism of erythrocyte destruction is probably unimportant in bovine trypanosomiasis.