C(1;Y)6/Dp(1;f)YP223 males that are homozygous for mei(2)yh151 show an increased frequency of non-disjunction of the fourth chromosomes (2.4%) but not of the sex chromosomes compared to wild type. Homozygous XY males show an increased frequency of non-disjunction of the fourth chromosomes (1.8%) but not of the sex chromosomes compared to wild type. Nondisjunction frequencies of the X and fourth chromosomes in homozygous females are comparable to those of controls.