Streptococcosis has been defined as a hemorrhagic septicemia in many fish species especially rainbow trout. This disease causes serious economic losses due to high mortality in fish and the bacteria responsible is also considered as a zoonotic pathogen. The aim of this study was identification of different causative agents of warm water streptococcosis and evaluation of the capsule gene in Lactococcus garvieae isolates collected from rainbow trout in Chaharmahal - Bakhtiari Province, Iran. A total of 430 clinically suspected fish were collected from 43 rainbow trout farms. The samples were analyzed using biochemical methods and multiplex-PCR. The results indicated that samples of 36 fish farms (83.7%) were infected with one of the studied bacteria. The infection by L. garvieae was found to be more than that by other species (30 of 36 farms, 83%). Infection by S. iniae and S. uberis was found in nine and one fish farm, respectively. Presence of virulence genes was studied in L. garvieae isolates by PCR assay. According to the results 34 of 50 isolates (68%) contained the capsule gene. The results revealed high prevalence of virulent L. garvieae in rainbow trout farms which should be considered in preventional policies such as vaccine production and also in public health.