Many bacterial pathogens encode ADP-ribosyltransferase toxins. The authors identified an ADP-ribosyltransferase toxin homologue (ArtA, ArtB) in Salmonella enterica serovar Typhimurium (S. typhimurium) DT104. ArtA is most homologous to a putative pertussis-like toxin subunit present in Salmonella typhi (STY1890) and Salmonella paratyphi A (SPA1609), while ArtB shows homology to a hypothetical periplasmic protein of S. typhi (STY1364) and S. paratyphi A (SPA1188), and a putative pertussis-like toxin subunit in S. typhi (STY1891) and S. paratyphi A (SPA1610). The artA gene was detected from the phage particle fraction upon mitomycin C induction, and the flanking region of artAB contains a prophage-like sequence, suggesting that these putative toxin genes reside within a prophage. Southern blotting analysis revealed that artA is conserved in 12 confirmed DT104 strains and in four related strains which are not phage-typed but are classified into the same group as DT104 by both amplified-fragment length polymorphism and pulsed-field gel electrophoresis. Except for one strain, NCTC 73, all 13 S. typhimurium strains which were classified into different groups from that of DT104 lacked the artA locus. The results suggest that phage-mediated recombination has resulted in the acquisition of art genes in S. typhimurium DT104 strains.