Proteus species are well-characterized opportunistic pathogens primarily associated with urinary tract infections (UTI) of humans. The Proteus O antigen is one of the most variable constituents of the cell surface, and O antigen heterogeneity is used for serological classification of Proteus isolates. Even though most Proteus O antigen structures have been identified, the O antigen locus has not been well characterized. In this study, we identified the putative Proteus O antigen locus and demonstrated this region's high degree of heterogeneity by comparing sequences of 40 Proteus isolates using PCR-restriction fragment length polymorphism (RFLP). This analysis identified five putative Proteus O antigen gene clusters, and the probable functions of these O antigen-related genes were proposed, based on their similarity to genes in the available databases. Finally, Proteus-specific genes from these five serogroups were identified by screening 79 strains belonging to the 68 Proteus O antigen serogroups. To our knowledge, this is the first molecular characterization of the putative Proteus O antigen locus, and we describe a novel molecular classification method for the identification of different Proteus serogroups.