SUMMARY: Determination has been made by the technique of abortive transduction of the functional relationships of over 200 Salmonella typhimurium histidine mutant alleles. The results are compared with those obtained by genetic recombination (Hartman, Loper & Šerman, 1960) and enzyme studies (Ames, Garry & Herzenberg, 1960). The results indicate that abortive transduction tests can be used to position rapidly sites of mutation on a chromosome map within the region of a single gene. The tests for genetic complementation reveal that some genes behave as single functional units, while other genes are composed of two, and some of four, functional subunits (complementation units). The existence of mutants with overlapping, intra-genic non-complementing patterns may allow crude mapping, solely by complementation, of some mutational sites within certain single genes. In gene D, the ordering of mutational sites by this technique so far conforms with that obtained by the more classic methods of map construction based on inferences derived from recombination analyses. Data are presented bearing on the individuality of the gene and the effect of mutation thereon. It is concluded that some genetic damages, localized within single genes, can secondarily affect the expression of adjacent gene(s) in addition to their primary effect upon the gene in which they are located (position effect).