A Legionella-like organism (strain 797-PA-H; ATCC 43130) was isolated from a specimen taken from an endotracheal tube of a patient 4 days before death and from the left lung at autopsy. Growth characteristics were consistent with those for Legionella species. Strain 797-PA-H gave negative test results with available direct immunofluorescence assay conjugates and with slide agglutination test antisera prepared against the 22 Legionella species and 35 serogroups now recognized. Minimal reactivity (1 to 2 +) was observed with both tests by using reagents prepared against the Legionella-like organism Lansing 3. Reciprocal absorption studies, however, showed that the cross-reactive antibodies could be removed easily. Physiologic, gas-liquid chromatographic, and DNA hybridization tests revealed that the strain belonged to the species Legionella pneumophila. Therefore, strain 797-PA-H was designated as the type strain of a new L. pneumophila serogroup, serogroup 11.