A bacterial strain, designated as Jin-1, was isolated from a water sample taken from the Jinyang Lake located in the suburbs of Taiyuan City, Shanxi Province. Its morphologic and tinctorial properties, nutritional requirements, growth characteristics, biochemical reactions, as well as results of DNA and peptide analysis overall met the criteria for identification of Legionella pneumophila. In serogrouping, extensive cross-reactions between Jin-1 and the type strains of L. pneumophila serogroup 1 through 14 were observed in both slide and tube agglutination tests mediated by the IgM antibodies. However, Jin-1 revealed the L. pneumophila serogroup 5 specificity in IFA, ELISA and dot-ELISA, all mediated by the IgG antibodies. Jin-1 was finally identified as a strain of L. pneumophila serogroup 5 with the more complex antigenic composition than the ATCC type strain of this serogroup. The taxonomic conclusion has been confirmed by CDC. A Legionella pneumophila strain which serologically cross-reacting with so many serogroups of this species has never been reported in the published articles. It is presumed that the cross-reacting antigen of Jin-1 belongs to the thymus-independent antigen by nature.