To eliminate this possibility, it will be necessary to map both the original mutant stock and mutant clones derived from it via lambda transduction. Summary. A preliminary map of the galactose region of E. coli K-12, obtained by the study of crossing over in heterogenotes showing position effect is presented. The order, kinase cistron-operator region-transferase cistron is indicated. The relationship of the epimerase cistron to the above region is not known, since it has as yet not been possible to study epimerase mutants.