The mutation am6 in the cI gene of bacteriophage lambda is identified as a C----T transition in a 5'CCATGG sequence. In four-factor crosses of am6 with nearby mutations in cI, the frequencies of cI+ recombinants are much higher than expected from the physical distances. A very short patch (VSP) mismatch repair system is presumed to recognize am6/am+ mispairs in the heteroduplexes that accompany recombination between the outside markers. Mutation am6 is corrected to am+; correction of am+ to am6 was not detected. Clear-plaque mutation 1-1 in cI is a T----C transition in a 5'CTTGG sequence, resulting in the sequence 5'CCATGG. When 1-1 was crossed with nearby mutations in gene cI, there were no excess cI+ recombinants, which would result from repair of CCTGG (1-1) to CTTGG (cI+). However, in crosses of cI+ phages with mutation 1-1, there was an excess of cI- recombinants, indicating that cI+ was repaired to 1-1. Preferential repair does not require adenine or cytosine methylation: when repairing a mismatch, the VSP repair system apparently identifies specific mispaired bases by sequence alone.