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Examination of the reported nucleotide sequences containing avirulence genes of Pseudomonas syringae pathovars suggested that avrA, avrB, avrC, avrPphC, avrRpm1, and avrPpiA1 are bordered by sequences similar to those of transposable elements of gram-negative bacteria. Repeat sequences and fragments of at least two different insertion sequence elements were identified at the ends of avrA and avrB, implying multiple transposition events for these areas. A DNA region homologous to a bacteriophage sequence was found upstream of avrPto. The linkage of various virulence/avirulence genes of animal- and plant-pathogenic bacteria with transposable elements and bacteriophage sequences, together with the presence of several of these genes on plasmids, supports the idea of horizontal transfer and frequent exchange of virulence/avirulence genes among bacterial pathogens.