Forty six pathological samples collected from the pig herds in Ho Chi Minh city, Dong Nai, Ba Ria - Vung Tau, Bac Giang provinces and Hanoi was determined the genotype of PRRS virus by nested RT-PCR. The results showed that 100 percent (46/46 samples) of PRRS virus infected belongs to North American genotype and only 2.18 percent (1/46 samples) PRRS virus infected belongs to European genotype. There is a multigenotype of PRRS virus infected in the studied pig herds: (i) infection with one genotype of Chinese North American PRRS virus (65.22 percent)
(ii) infection with one genotype of classical North American PRRS virus (32.60 percent)
(iii) infection with two genotypes of North American and European PRRS virus (1.28 percent). Thus, the results of this study presented the complexity of PRRS virus infection genotype in the Vietnamese pig herds. The complexity of PRRS infection genotype cause the difficulties not only in diagnosis but also in vaccine application to control the PRRS.