To determine the frequency and characteristics of reassortment among Hantaan and Seoul viruses causing hemorrhagic fever with renal syndrome (HFRS).Mixed infections were initiated in tissue culture, using Hantaan virus strain 76 - 118 and Seoul virus strain SR-11. Potential reassortant virus plaques were picked out by multiplex RT-PCR, using primers specific for individual genome segments (L, M, S) of each strain.Most of the progeny virus plaques (68.19% of 44) had parental genotype of 76 - 118 strain or SR-11 strain while 2 of 44 plaques had mixed genotypes that yielded RT-PCR bands for the same segment of both parental strains. Reassortant viruses were detected in 68.19% of 44 progeny plaques tested, involving the M and S segments. In addition, approximately 4.55% of the progeny virus plaques appeared to contain S or M segments originating from both parental virus strains, showing that they were diploid.Genetic reassortment can occur between Hantaan virus and Seoul virus strains.