Conjugative plasmid transfer systems have been well studied, but very little is known about the recipient factors that control horizontal transmission. A self-transmissible IncP-9 naphthalene catabolic plasmid, NAH7, carries the traF gene, whose product is considered to be a host-range modifier of NAH7, because its traF deletion mutant (NAH7dF) is transmissible from Pseudomonas putida to P. putida and Escherichia coli and from E. coli to E. coli, but not from E. coli to P. putida. In this study, transposon mutagenesis of P. putida KT2440 was performed to isolate the mutants that could receive NAH7dF from E. coli. The mutants had the transposon insertions in ptsP or ptsO, encoding two of three components of the nitrogen-related phosphotransferase system (PTSNtr). The KT2440 derivative lacking ptsN, encoding the remaining component of PTSNtr, was also able to receive NAH7dF. These results indicated that the PTSNtr in P. putida is involved in inhibition of conjugative transfer of NAH7dF from E. coli. Our further experiments using site-directed mutants suggested the indirect involvement of the phosphorylated form of PtsO in the inhibition of the conjugative transfer. Conjugative transfer of NAH7 and another IncP-9 plasmid, pWW0, from E. coli was partially inhibited by the PtsO function in KT2440.