Schizosaccharomyces pombe cdc16p is required to limit the cell to forming a single division septum per cell cycle; the heat-sensitive loss-of-function mutant cdc16-116 completes mitosis, and then undergoes multiple rounds of septum formation without cell cleavage. cdc16p is a homologue of Saccharomyces cerevisiae BUB2p, and has also been implicated in the spindle assembly checkpoint function in S. pombe. To identify other proteins involved in regulating septum formation, we have screened for multicopy suppressors of the cdc16-116 mutation. In this paper, we describe one of these suppressors, zfs1. The null allele (zfs1-D1) is viable. However, at low temperatures it divides at a reduced size, while at higher temperatures, it partially suppresses heat sensitive mutants in genes signalling the onset of septum formation. Zfs1-D1 cells show an increased rate of chromosome loss during exponential growth. Moreover, if assembly of the spindle is prevented, zfs1-D1 cells do not arrest normally, but the activity of cdc2p kinase decays, and cells form a division septum without completing a normal mitosis. We conclude that zfs1 function is required to prevent septum formation and exit from mitosis if the mitotic spindle is not assembled. The suppression of cdc16-116 by zfs1 is independent of dma1 function and the spindle assembly checkpoint genes mad2 and mph1. The genetic interactions of zfs1 with genes regulating septum formation suggest that it may be a modulator of the signal transduction network controlling the onset of septum formation and exit from mitosis.