We studied the timing of mitotic loss of maternally and paternally derived chromosomes among the progeny of Drosophila melanogaster females homozygous for an amorphic mutation in ncd, a gene encoding a kinesin-like protein. In order to determine the division at which chromosome loss occurs, we estimated the fraction of XO nuclei resulting from X chromosome loss by scoring the phenotype of 47 adult cuticular landmarks in 160 XX-XO mosaics (gynandromorphs) derived from maternal X chromosome loss, and 33 gynandromorphs derived from paternal X chromosome loss. The results show that while most of the mitotic loss of maternally derived chromosomes occurs at the first cleavage division, the mitotic loss of paternally derived chromosomes occurs only at the second and later divisions. This means that paternally derived chromosomes are immune from the effects of ncd prior to karyogamy, which occurs after the first cleavage division. We discuss the implications of these results for the function of the ncd gene product and for other kinesin-like proteins in Drosophila.