To study the determinants which are involved in the loss of pathogenicity in wilt-inducing Pseudomoms solamcewum, several physlologica I functions were compared in a virulent P. solanacearum strain and an avirulent, spontaneously derived mutant strain. The polyacrylamide gel electrophoresis showed the distinction between two strains in the patterns and the relative intensity of proteins produced intracellularly or extracellularly. Enzyme assays showed that the level of polygalacturonase activity in the culture filtrate of the avirulent mutant was markedly reduced, while carboxymethylcellulase(rondoglucanase) activity in both strains were nearly negligible. These results suggest that the loss of pathogenicity in mutant strain is attributed in part to the reduced production of polygalacturonase. In audit ion, comparative analyses by agarose gel electrophoresis of DNA molecules isolated from both strains show that the pathogenicity genes of p. solanaceerum are not located on plasmid but are on chromosome.