Mitosis in S. cerevisiae depends on IPL1 kinase, which genetically interacts with GLC8. The metazoan homologue of GLC8 is inhibitor-2 (I-2), but its function is not understood. We found endogenous and ectopic I-2 localized to the spindle, midzone and midbody of mitotic human epithelial ARPE-19 cells. Knockdown of I-2 by RNAi produced multinucleated cells, with supernumerary centrosomes, multipolar spindles and lagging chromosomes during anaphase. These defects did not involve changes in levels of PP1C, and the multinuclear phenotype was rescued by overexpression of I-2. Multiple nuclei and supernumerary centrosomes required progression through the cell cycle and I-2 knockdown cells failed cytokinesis, as observed by time-lapse microscopy. Inhibition of Aurora B by hesperadin produced multinucleated cells and reduced H3S10 phosphorylation. I-2 knockdown enhanced this latter effect. Partial knockdown of PP1C alpha prevented multiple nuclei caused by either knockdown of I-2 or treatment with hesperadin. Expression of EGFP-I-2 or HA-I-2 made cells resistant to hesperadin. We propose that I-2 acts to enhance Aurora B by inhibiting specific PP1 holoenzymes that dephosphorylate Aurora B substrates necessary for chromosome segregation and cytokinesis. Conserved together throughout eukaryotic evolution, I-2, PP1 and Aurora B function interdependently during mitosis.