In order to find out the cause of the massive death of largemouth bass （Micropterus salmoides） juveniles in a breeding farm in Shaoguan City， Guangdong Province， clinical observation， and bacteriological， parasitological and virological detection of the affected fish were carried out. And after excluding the possibility of bacterial and parasitic infections， RT-PCR detection， cell isolation and culture， artificial infection， hematoxylin-eosin （H&E） staining of histopathological sections， transmission electron microscope observation of ultrathin sections and phylogenetic tree analysis were performed and a Micropterus salmoides rhabdovirus （MSRV） was isolated and named as the Micropterus salmoides rhabdovirus Shaoguan strain （MSRV-SG01）. The results of the artificial infection test showed that the experimental fish died within 2 days， accompanied by clinical symptoms such as hemorrhage， rotten tail， and dragging stools， and the mortality rate reached 100% within 7 days. Histopathological sections showed that the liver and spleen of the diseased fish showed extensive necrosis， which was consistent with the symptoms of naturally diseased fish. According to the amino acid sequence of protein G， the MSRV-SG01 strain isolated in this study was compared with other rhabdoviruses reported in GenBank by phylogenetic tree analysis and the result showed that FJ985， MSRV-YH01， SCRV， and SCRV are grouped together， and the similarity with the reported MSRV-FJ985 and MSRV-YH01 strains were higher than 97%. Through the above experimental analysis， it was determined that the cause of mass death of largemouth bass juveniles was MSRV infection.