An R plasmid, R100-1, was mapped previously (Yoshikawa, 1974) by transduction from an integratively suppressed Hfr strain to a recipient with a mutation in gene dnaA. By this method various types of transductants of plasmid R100-1 that exist autonomously or in the integrated state were obtained. Seventy-one such transductants were used in the present study to map gene inc, which is responsible for incompatibility. The results obtained can be explained by either of the following: (i) R100-1 has only a single gene or gene cluster (inc) despite previous work suggesting that this plasmid is a co-integrate of two replicons; (ii) R100-1 possesses more than one inc locus located between the repA and tra loci.