The sex chromosome bivalent was examined in Drosophila melanogaster males possessing the In(1)sc4Lsc8R X chromosome. Three-dimensional reconstructions from electron micrographs of serially cut thin sections were made. A large proportion of the kinetochores of In(1)sc4Lsc8R/Y bivalents did not face opposite poles during metaphase I and anaphase I. This suggests that In(1)sc4Lsc8R/Y bivalents may have difficulty achieving bipolar stability. Delay in achieving bipolar stability could contribute to the nondisjunctional behavior found in In(1)sc4Lsc8R/Y males.