1. The sex ratio in pure-line cultures of strains of a white-eyed mutant ofCalliphora erythrocephala varied from strain to strain. The average for 49, 161 flies was 46% females and 54% males. Different strains varied in the ratio of females to males. The lowest was 40% females to 60% males and the highest 50% females to 50% males. 2. In cultures of wild-typeC. erythrocephala it was found that on the average more males than females were produced. Of 27,880 flies, 46% were females and 54% males. Different strains gave different ratios of females to males, varying from 50% females to 50% males to 45% females to 55% males. 3. During extensive inbreeding experiments with pure-line cultures a number of possible mutations occurred, including some mosaics in which only part of the insect showed the mutant character. 4. In two cases when females heterozygous for the white-eyed mutant gene were crossed with males bearing the mutant gene in theirX-chromosome, the progeny consisted of wild-type males and only wild-type females, instead of white-eyed and wild-type females and males in the proportion 1∶1∶2. Possibly these cases were due to the fertilization of the parent females by males with non-disjunction of theXY-chromosomes.