The extent of the deletions of five Tets Tra- mutants of R100-1 was determined by complementation experiments with wild-type and tra mutants of Flac. The presence or absence of the origin of transfer on the mutants was also investigated. Using the results, a tentative map of this region of the R factor was drawn: it was essentially similar to the analogous region of the E. coli K12 F factor, except that tet was located between traJ and traA. Some of the deletions had removed the promoter for the transfer operon. This allowed detection of the transcription of traC and distal genes from a weak, traJ-independent promoter. This is probably the Is2 promoter, since R100-1 carries and Is2 insertion sequence located immediately to the left of traC in the correct orientation. Since neither the transfer operon promoter nor the Is2 promoter seemed to be required for transcription of traI, it was concluded that, unlike the F factor, this was located in a separate operon.