The genes xthA, pncA, and pabB were ordered relative to others by two- and three-factor transductional crosses with bacteriophage P1. The genes studied span 2 min (2%) of the genetic map of Escherichia coli K-12 in the clockwise sequence pheS-pfkB-xthA-pncA-gap-pabB-fadD. Eleven independently derived xth mutations were examined; all were known to affect exonuclease III and its associated endonuclease II activity, and all were mapped in the xthA region. pncA mutations were found to confer resistance to 6-aminonicotinamide, whereas some pheS mutations are known to specify resistance to p-fluorphenylalanine. xth mutations were readily transferred into other strains by selecting for these co-transducible drug resistance markers.