Aims: To validate identification methods for Legionella pneumophila strains that cannot be serotyped into the known serogroups and to characterize their antigenic diversity. Methods and Results: Fifty L. pneumophila strains that could not be serogrouped, but which had been confirmed as L. pneumophila by mip gene sequencing, were further identified phenotypically. We used (i) MONOFLUO anti-Legionella Staining Reagent (Bio-Rad) (50/50), (ii) an in-house prepared immunoblot assay for the detection of L. pneumophila-specific Mip protein epitope (50/50), (iii)fatty acid analysis using the Microbial Identifications System (MIDI) (47/50) and (iv) Oxoid agglutination tests (44/50). The serological diversity was further characterized by testing with five serogroup-cross-reactive monoclonal antibodies, resulting in nine phenons. Conclusions: The division of L. pneumophila into 15 serogroups does not reflect the serogroup heterogeneity. Results of these tests indicate that there are more serogroups. Significance and Impact of the Study: MONOFLUO anti-Legionella Staining Reagent is the only commercially available tool for identifying atypical strains of L. pneumophila. If necessary for epidemiological purposes, the antigenic heterogeneity of these strains can be analysed by monoclonal antibodies.