In crosses under rec+, red+, gam+ conditions, mutation am6 in the cI (repressor) gene of bacteriophage lambda recombines with other cI mutations much more frequently than predicted by the physical distances involved. In four-factor crosses of am6 with mutations located 22-60 base pairs to the left, cI+ recombinants that are expected to require three crossovers (triple recombinants) are more frequent than recombinants that require only one crossover. However, when am6 is crossed with large insertions in cI, which may be expected to interfere with the formation of heteroduplexes by branch migration, the frequency of cI+ triple recombinants is very low. In addition, cI+ recombinants in crosses between am6 and adjacent mutations have a high probability of retaining the flanking markers of the am6 parent. These findings suggest that am6 is particularly susceptible to mismatch repair in heteroduplexes spanning cI. A large fraction of such heteroduplexes are presumed to be the result of branch migration from crossovers occurring at some distance from am6. The absence of co-repair when am6 is crossed with adjacent cI mutations indicates that most repair tracts extend no farther than about 20 bp to either side of the mismatch. The am6 mutation arose in the glutamine codon in a CCAGG sequence, in which the central cytosines are methylated in K12 strains. Their location in methylated sequences may make certain amber mutations susceptible to a specific very short patch (VSP) repair.