Esamining 12 plasmids in Pseudomonas aeruginosa, we found two types of interaction in their transfer (inhibition and facilitation), using donor cells carrying two compatible plasmids. (i) Ten plasmids representing incompatibility groups P-1, P-2, P-5, P-6, and P-7 were all transmissible at a high frequency, 10-2 to 10-1, except for one with a lower frequency of about 10-3. The transfer of P-5 plasmids was inhibited by P-2 plasmids reciprocally or unilaterally, and the unilateral transfer inhibition was observed in other combinations between plasmids belonging to groups P-1, P-2, P-6, and P-7. It was characteristic of Pseudomonas plasmids that most plasmids with high transferability inhibited the transfer of other coexisting plasmids without distinct inhibition of their own transfer. (ii) Two plasmids, Rms149 of P-8 group and Rlb679, which was not classified, were transmissible at an exceptionally low frequency of 10-7 to 10-6, but their transfer was facilitated by plasmids with high transferability.