Using a strain containing a cir-lac operon fusion and a selective medium, we isolated a regulatory mutant of the colicin I receptor, which we have designated cirR. Cells carrying the cirR mutation were defective in the transcriptional regulation of cir by iron, but synthesis of other iron-regulated proteins was unaffected. cirR was found to be cis dominant. This is in contrast to previously described mutations in iron regulation which are pleiomorphic and trans dominant. Temperature regulation of colicin I receptor production was unaffected by cirR.