1. Free-living populations ofD. melanogaster andD. subobscura show a considerable incidence of heterozygosis for autosomal recessives. In 1933 frequencies wereD. subobscum 0.277 ±0.117,D. melanogaster 0.177±0.100, and in 1934D. subobscum 0.636 ± 0.149 andD. melanogaster 0.924 ± 0.251. Differences between successive years are largely accounted for by improvements in technique. 2. InD. subobscura no sex-linked mutants were found, and inD. melanogaster only one. No dominants, sex-linked or autosomal, were found in either species; one mutant, shaven, inD. subobscura exhibited incomplete dominance only after selection for high expression. 3. The realisation of mutants was noted and a coefficient of realisation of each mutant was calculated. A great many mutants of low realisation were found. In many cases the low realisation was shown to be due to the action of genes which modified mutants back to wild-type. 4. Of the mutants found in 1933 inD. melanogaster two are hitherto unrecorded, one is a more intense allelomorph of cinnabar, one an allelomorph in the ebony series, and one, rotated abdomen, at the same locus as the rotated, abdomen already known. The investigation of the identity of the 1934 mutants is not yet completed.