Streptococcus suis (S. suis) serotype 2 is an important pathogen to cause bacterial meningitis in adult patients in Vietnam. S. suis is commonly found on pig tonsil and can cause diseases in swine. The risk factors for human infection with S. suis includes frequent exposure to pigs and consumption of S. suis contaminated pork products. Pig co-infected with porcine reproductive and respiratory syndrome virus (PRRSV) and S. suis serotype 2, in experiment animal models, showed the increase of severity in outcome. However, data of systematic field study on secondary infection of S. suis in PRRSV infected pigs was never been investigated. the authors proposed to investigate the secondary infection of S. suis serotype 2 in PRRSV infected pigs which were reared in Thai Binh province during the outbreak of PRRS. Results of reverve transcriptase real time PCR confirmed the infection of PRRSV in 12 (96.15 percent) families. Secondary infection of S. suis in these pigs were investigated by bacteriological culture and DNA amplification. DNA of S. suis serotype 2 was detected in blood specimens of 32 (94 percent) PRRSV infected pigs in these farms. This high infection rate suggested it is important that persons who directly contact with pigs like farmers, sick pig care takers: veterinarian involving in treating and culling sick pigs should practise biosafety strictly and fully to miniihize the risk of S. suis infection, specially during PRRS outbreaks.