C(1;Y)6/Dp(1;f)YP223 males that are homozygous for mei(2)yh1491 show a slightly increased frequency of non-disjunction of the sex chromosomes (2.3%) and fourth chromosomes (2.4%) compared to wild type (0.8 and 0.1% respectively). Homozygous XY males show a slightly increased frequency of non-disjunction of the sex chromosomes (1.2%) and fourth chromosomes (1.0%) compared to wild type (0.1 and 0.3% respectively). Analysis of the progeny of homozygous males mated to compound-X females indicates that mei(2)yh1491 induces either nondisjunction at meiosis II or chromosome loss at meiosis I and/or II. Nondisjunction frequencies of the X and fourth chromosomes in homozygous females are comparable to those of controls.