In order to determine the causes of tilapia disease in a farm in Dongguan City，and to develop targeted countermeasures，brain，liver，spleen，kidney and other tissue samples were collected from diseased tilapia for bacterial isolation，culture and identification，followed by biochemical identification using VITEK-2 Compact，16S rRNA gene sequence analysis and construction of a phylogenetic tree，and finally antimicrobial susceptibility test on the isolated strain（DG210721）. The results showed that the characteristics，morphology and biochemical identification results of the obtained strain were consistent with those of streptococcus agalactiae；it was found that，after 16S rRNA gene sequence analysis，the homology between the isolated strain and streptococcus agalactiae（JQ039371）was up to 100%，with closest genetic relationship；the isolated strain，from strong to weak，was sensitive to doxycycline hydrochloride，enrofloxacin，thiamphenicol，florfenicol，fluoromequine，sulfamethoxazole+trimethoprim，sulfamethoxam sodium and neomycin sulfate. In conclusion，streptococcus agalactiae infection was one of the causes leading to tilapia disease in the farm，and the isolated strain，with stable genetic sequence，was resistant to some clinical drugs to some extent. Therefore，it was necessary to further prevent and control streptococcus agalactiae infection in tilapia，regularly sample for detection，diagnose in time，and scientifically prevent and control the disease using sensitive drugs.