Abstract: A strain of bacteria was isolated from the attacked tilapia. The pathogen was studied after isolation and purification. A drug sensitivity test showed that the pathogen was sensitive to 10 drugs containing chloramphenicol.The cfb gene was cloned and sequenced.The biological characteristics of the pathogen were studied.The test included many aspects, such as effect of cultivating time and pH on the growth, effect of cultivating temperature on the virulent,the relationships between disease outbreak rate and attacking manners, susceptibility of different specifications(100 and 10 g)fish to the bacteria. The results indicated that the pathogen was Streptococcus agalactiae of tilapia, and the best time and pH for cultivating was 11.5 h and 7.5, respectively. The onset of three injection methods (peritoneal, pleural and back muscle) was the same, but the slowest death rate was the one injected by pleural. Back muscle injection was safer to youngfish. The fish was attacked fastest and had the most centralized death rate when cultivating temperature was 33℃. The 100 g fish was easier to be attacked than the 10 g fish.

Key words: tilapia; Streptococcus agalactiae; drug sensitivity; biological characteristics; PCR; pathogenicity