Amber and missense mutations in gene cI of bacteriophage lambda were mapped by reciprocal four-factor crosses, selecting recombinants between the outside markers (N amber and O amber). Distances between cI missense mutations were additive. Several cI amber mutants recombined with other cI mutations with a higher frequency than expected from the map location. Multiple exchanges in the N-O region occurred at a frequency greater than expected by chance. This "high negative interference" was especially marked in crosses with the cI amber mutations that were strong recombiners. A new ind mutation, ind2, was found near tsU51, to the left of the previously-known ind1 mutation, which is located almost in the center of gene cI. The mutation c50 maps to the right of tsU50 and c71. Mutations c60, and ts71, which differ in phenotype, are apparently at the same site.