39 tonB i- deletions were used to map 57 independently isolated i gene mutants. Methods selective for i+ recombinants from various types of i mutants are described. i+ recombination frequencies of 6 × 10-6 to 6 × 10-7 can be detected in the different selective systems. Twenty is mutations and 12 i-d mutations map in distinct but different regions of the gene. i-sus mutations are scattered over the gene.