We have purified the CI repressor protein of bacteriophage phi 80. Its N-terminal amino acid sequence and its amino acid composition agree with those predicted from the nucleotide sequence of the cI gene. The phi 80 CI repressor was cleaved at a Cys-Gly bond by the wildtype RecA protein in the presence of single-stranded DNA and ATP or its analogues. This cleavage site is different from other repressors such as LexA, lambda CI and P22 C2, which were cleaved at an Ala-Gly bond. The phi 80 CI repressor was cleaved at the same site by the RecA430 protein, but was not cleaved by the RecA1 protein. This effect of the bacterial recA mutations on cleavage is consistent with the fact that prophage phi 80 in recA430 cells can be induced by irradiation with ultraviolet light, while the prophage in recA1 cells cannot.