C(1;Y)6/Dp(1;f)YP223 males that are homozygous for mei(2)yh217¹ show 4.4% non-disjunction of the sex chromosomes and 1.0% non-disjunction of the fourth chromosomes compared to 0.8% and 0.1% in wild type respectively. Homozygous XY males show 2.5% non-disjunction of the sex chromosomes and 0.3% non-disjunction of the fourth chromosomes compared to 0.1% and 0.3% in wild type respectively. Analysis of the progeny of homozygous males mated to compound-X females indicates that mei(2)yh217¹ specifically affects meiosis I. In addition, the mutant shows meiotic drive (a discrepancy in the recovery of reciprocal products of meiotic segregation); the recovering of X-bearing sperm exceeded that of Y-bearing sperm and the recovery of nullo-XY sperm exceeded that of XY sperm. Homozygous female lay a large number of eggs with normal eggshell morphology, but nuclear divisions become abnormal by the syncytial blastoderm stage.