On the basis of the hypothesis that mutants in genes controlling essential cell cycle functions in Drosophila should survive up to the larval-pupal transition, 59 such 'late lethals' were screened for those mutants affecting cell division. Examination of mitosis in brain neuroblasts revealed that 30 of these lethals cause disruptions in mitotic chromosome behavior. These mutants identify genes whose wild-type functions are important for: (1) progression through different steps of interphase, (2) the maintenance of mitotic chromosome integrity, (3) chromosome condensation, (4) spindle formation and/or function, and (5) completion of cytokinesis or completion of chromosome segregation. The presence of mitotic defects in late lethal mutants is correlated tightly with the presence of defective imaginal discs. Thus, the phenotypes of late lethality and poorly developed imaginal discs are together almost diagnostic of mutations in essential cell-cycle functions. The terminal phenotypes exhibited by these Drosophila mitotic mutants are remarkably similar to those observed in mammalian cell-cycle mutants, suggesting that these diverse organisms use a common genetic logic to regulate and integrate the events of the cell cycle.