AbstractOne thousand lines of a balanced lethal strain of Drosophila melanogaster heterozygous at seven allozyme loci were subjected to chronic (0.15 rad/min) γ‐irradiation (137 Cs). After 15 generations of exposure they were screened by gel electrophoresis for newly arisen null mutants and/or mobility variants. Seven independent nulls were recovered. 1) None were associated with a cyto‐genetically detectable aberration; 2) one of six analyzed was associated with recessive lethality, suggesting association with a submicroscopic interlocus deficiency; 3) three of five which could be analyzed for cross‐reacting material (CRM) production were CRM‐positive; and 4) two of six produced an active heterodimer with an active normal allele. The results taken together suggest that most of the mutations are point mutations rather than small deficiencies.