Streptomycin resistance in strains of Pseudomonas syringae pv. papulans, Pantoea agglomerans and a yellow-pigmented, non-fluorescent Pseudomonas sp. (Py), isolated from apple orchards in New York and Washington states, is predominantly associated with strA-strB genes carried on conjugal plasmids (R plasmids). None of 128 resistant Erwinia amylovora strains from the eastern and wester USA hybridized with a strA-strB prove, SMP3. Resistant Py strains transfered R plasmids to Ps. syringae pv. papulans and to Py in vitro at frequencies of 10-1-10-2 per recipient cell whereas Ps. syringae pv. papulans transferred its plasmids at frequencies of 10-2 to below detectable levels. Transfer of R plasmids to P. agglomerans was not detected and resistant P. agglomerans did not transfer their R plasmids to any recipients. R plasmids were found to be highly diverse as measured by DNA fingerprint analysis. Transfer-deficient transposon mutants of R plasmid pCPP519 were generated, and 3·9 kb EcoRI and 3·0 kb SmaI fragments that hybridized with a Tn5 probe were cloned and sequenced. The deduced amino acid sequences of the 3·9 kb fragment were similar to proteins involved in replication, nicking at oriT, and piliation in other bacteria.