Abstract From the ca;px stock, which is the progenitor of Om mutants caused by insertions of the tom retrotransposon, 50 kb of genomic DNA including the Om(1D) locus was cloned by tom tagging and chromosome walking. Southern blot analyses of six Om(1D) mutants exposed one or two tom elements inserted at five nonrandom sites within an 18-kb distal segment of the restriction map; the phenotypic uniformity between these mutants was not affected by variations in the position, number or orientation of their inserts. Spontaneous revertants or more extreme derivatives of Om(1D) alleles were nonlinearly associated with losses or gains of tom inserts. Seven of eight radiation induced derivatives of Om(1D) mutants had one breakpoint of a chromosome rearrangement in polytene section 13A which includes the Om(1D) locus. Two Om(1D) derivatives, a spontaneous revertant and an induced extreme allele, were associated with overlapping deficiencies which define a region that is likely to contain the Om(1D) coding seguences proximal to the tom insertion sites. Incidental results confirm the previously indicated homology of the Om(1D) locus with the Bar locus of Drosophila melanogaster.