Pseudomonas aeruginosa strain GT424, derived by the transfer of plasmid R45 to strain GT1, was shown to have an Era+ phenotype (enhanced recipient ability) for enterobacterial plasmids from incompatibility (Inc) groups C, FI, FII, J, N, W, and X. Increase in transfer frequency ranged from 10 to 10(6)-fold. Plasmids of IncFII, IncJ, and IncX were found to be transmissible only to the Era+ strain. Plasmids of Inc groups A-C, M, and T transferred at low frequency (less than 10(-7)) to both GT1 and GT424 and did not respond to the Era+ character of the latter. Also not responsive to the Era character were plasmids R144drd3 (IncI alpha) and RP1 (IncP), which transferred to both GT424 and GT1 at intermediate (10(-4)) and high frequencies (10(-1)), respectively. All plasmid-mediated antibiotic resistances that could be measured as well as UV resistance and susceptibility were found to be expressed in P. aeruginosa. Plasmid-coded phage susceptibilities, however, generally were not phenotypically manifested in the P. aeruginosa strains. The physical structure of plasmids in P. aeruginosa could be classified into four major types. Some of the plasmids underwent additions or deletions and were unstable; the majority, however, appeared to integrate into the chromosome. Some members of IncN, IncC, IncJ, and IncP were found to be transmissible from P. aeruginosa back to Escherichia coli. This is the first report of the successful transfer to P. aeruginosa of plasmids from many of the Inc groups that have been defined in Enterobacteriaceae, namely, FI, I alpha, J, M, T, and X.