The Salmonella genus is currently divided into 2 species, Salmonella bongori and Salmonella enterica, the latter being classified into 6 subspecies, S. enterica subsp. enterica, salamae, arizonae, diarizonae, houtenae, and indica (1). Most salmonellae possess two flagellin genes, fliC and fljB, encoding the so-called phase 1 and phase 2 flagellins, respectively. Only one flagellin type is expressed at a time in a Salmonella cell. Isolates are termed biphasic, monophasic, or nonmotile if they can express both, one, or no flagellar phases, respectively. Expression of the two genes is under the control of a switch mechanism called "phase inversion." The switch mechanism consists of the DNA inversion of a sequence segment, the so-called invertible region, by recombination of the two flanking sites, hixL and hixR, mediated by an invertase encoded by the hin gene included in this segment (2). The promoter of the fljBA operon is born on this invertible region. The fljBA operon encodes the FljB flagellin and FljA, a repressor of the fliC gene. The orientation of the invertible region determines whether the fljBA operon or the fliC gene will be expressed (2).Characterization of Salmonella spp. below the subspecies level is traditionally obtained by serotyping based on the KauffmannWhite-Le Minor scheme (1). Serovars are determined following agglutination of entire bacteria with specific sera to identify variants of the somatic (O) and flagellar (H) antigens. Serotyping results in an antigenic formula, indicating the O, H1, and H2 antigens separated by colons. The different O and H antigens are named by numbers, alphabetical characters, or both. Some O antigens can be present or absent without interfering with serovar identification. These antigens are interesting only as epidemiological markers within a given serovar. These accessory O antigens are underlined when their presence is linked to the integration of a prophage. O or H antigens are written within square brackets if they may be present or absent without relation to phage conversion. The 1,4,[5],12:i:1,2 antigenic formula, corresponding to S. enterica serovar Typhimurium, therefore indicates that (i) the O4 and O12 antigens are always present in this serovar, (ii) the O1 antigen can be present or absent due to phage conversion, (iii) the O5 antigen can be present or absent without relation to phage conversion, and (iv) the H1 antigen "i" and the H2 antigens "1" and "2" are always present. If no H1 or H2 antigen can be detected,