Strains of Pseudomonas syringae pv. syringae were isolated from healthy and diseased stone fruit tissues sampled from 43 orchard sites in California in 1995 and 1996. These strains, together with P. syringae strains from other hosts and pathovars, were tested for pathogenicity and the presence of the syrB and syrC genes and were genetically characterized by using enterobacterial repetitive intergenic consensus (ERIC) primers and PCR. All 89 strains of P. syringae pv. syringae tested were moderately to highly pathogenic on Lovell peach seedlings regardless of the host of origin, while strains of other pathovars exhibited low or no pathogenicity. The 19 strains of P. syringae pv. syringae examined by restriction fragment length polymorphism analysis contained the syrB and syrC genes, whereas no hybridization occurred with 4 strains of other P. syringae pathovars. The P. syringae pv. syringae strains from stone fruit, except for a strain from New Zealand, generated ERIC genomic fingerprints which shared four fragments of similar mobility. Of the P. syringae pv. syringae strains tested from other hosts, only strains from rose, kiwi, and pear generated genomic fingerprints that had the same four fragments as the stone fruit strains. Analysis of the ERIC fingerprints from P. syringae pv. syringae strains showed that the strains isolated from stone fruits formed a distinct cluster separate from most of the strains isolated from other hosts. These results provide evidence of host specialization within the diverse pathovar P. syringae pv. syringae.