The samples of tilapia and red tilapia were collected from the disease outbreaks in tilapia culture areas of Ho Chi Minh city and some nearby provinces for isolation and identification of bacteria. The studied result showed that Streptococcus was found in 26 (41.16 percent) samples (out of 63 samples). Of which, S. agalactiae accounted for 84.16 percent, following by S. difficilis (11.54 percent) and S. iniae (3,85 percent). None of S. uberis was identified from the isolated samples. The results of bio-chemical test indicated that there was different on biochemical characteri'stics amongst the isolated Streptococcus strains in the same species. The result of virulent test indicated that all of the isolated Streptococcus strains caused 40 80 percent of fish died with a challenge dose of 1-2 x 10 exponent 7 cfu per fish.