In animal cells, cytokinesis begins shortly after the sister chromatids move to the spindle poles. The inner centromere protein (Incenp)has been implicated in both chromosome segregation and cytokinesis, but it is not known exactly how it mediates these two distinct processes. We identified two Caenorhabditis elegans proteins, ICP-1 and ICP-2, with significant homology in their carboxyl termini to the corresponding region of vertebrate Incenp. Embryos depleted of ICP-1 by RNA-mediated interference had defects in both chromosome segregation and cytokinesis. Depletion of the Aurora-like kinase AIR-2 resulted in a similar phenotype. The carboxy-terminal region of Incenp is also homologous to that in Sli15p, a budding yeast protein that functions with the yeast Aurora kinase Ipl1p. ICP-1 bound C. elegans AIR-2 in vitro, and the corresponding mammalian orthologs Incenp and AIRK2 could be co-immunoprecipitated from cell extracts. A significant fraction of embryos depleted of ICP-1 and AIR-2 completed one cell division over the course of several cell cycles. ICP-1 promoted the stable localization of ZEN-4 (also known as CeMKLP1), a kinesin-like protein required for central spindle assembly. ICP-1 and AIR-2 are part of a complex that is essential for chromosome segregation and for efficient completion of cytokinesis. We propose that this complex acts by promoting dissolution of sister chromatid cohesion and the assembly of the central spindle.