During a survey of parasitic disease,a species of Argulus was isolated from the cultured largemouth bass (Micropterus salmoides) at a fish farm in Wuhan,China. Morphological and morphometric analysis were conducted to identify the species and phylogenetic analysis of Argulus were carried out based on SSU rDNA. The results showed that the parasite was in faint yellow. The total length was 1.93±0.46 (1.42-2.54) mm for female and 2.18±0.49 (1.57-2.87) mm for male. The carapace was horseshoe shaped,with branched pigments scattered throughout the lobes. The sucker consisted of chitinous strips (female:44-52,male:46-47),each of which contains varying amounts of chitinous tablets (female:5-6,male:7-8). Unreleased eggs were founded in the abdomen of female and symmetrical testis were observed in the male’s abdomen. Morphological and morphometric data was in consistent with Argulus japonicus Thiele 1900. Phylogenetic analysis based on ML phylogenetic tree of SSU rDNA showed that Argulus species was divided into three clades. A. japonicus isolates were mainly clustered into Clade I and paraphyletic grouped with Argulus foliaceus and Argulus rhipidiophorus. In conclusion,we identified the Argulus sp. isolated from the largemouth bass as A. japonicus based on the above morphological and morphometric data. Further phylogenetic analysis was also conducted to discuss the phylogenetic relationship within Argulus. To our knowledge,this is the first report of the parasitism of A. japonicus on largemouth bass.