Five different field-collected isolates (i.e., Anhui, Hubei, Guangxi, Sichuan and Yunnan) of Schistosoma japonicum from the mainland of China were compared by means of multilocus enzyme electrophoresis in polyacrylamide gels of individual male worm extracts. The enzyme systems used in the study included GDH, G6PD, LDH, MDH, PGI, PGM and SOD. Of 9 loci examined in the 7 isozyme systems, 4 were found to be polymorphic. These were LDH-1, LDH-2, MDH and PGM, the remainder of GDH, G6PD-1, G6PD-2, PGI and SOD being monomorphic. Our results indicated that the genetic polymorphism occurring in the natural population of different isolates of S. japonicum in the mainland of China. The allele frequencies at polymorphic loci coding for enzyme as well as the genetic identity and genetic distance in different isolates of S. japonicum will be reported elsewhere.