SUMMARY: Of 12 R factors transferred from wild strains of Proteus rettgeri to Escherichia coli K12, all were fi - Five were of the N-compatibility group; three belonged to a newly defined group T, of which Rts1 is the prototype; one belonged to group W. The other three constituted at least one compatibility group, not previously described. The replication of two of the T plasmids, like that of Rts1, was temperature sensitive. This set of R factors is significantly different from those derived from bacteria of other genera.