In the filamentous fungus, Aspergillus nidulans, multiple rounds of nuclear division occur before cytokinesis, allowing an unambiguous identification of genes required specifically for cytokinesis. As in animal cells, both an intact microtubule cytoskeleton and progression through mitosis are required for actin ring formation and contraction. The sepH gene from A. nidulans was discovered in a screen for temperature-sensitive cytokinesis mutants. Sequence analysis showed that SEPH is 42% identical to the serine-threonine kinase Cdc7p from fission yeast. Signalling through the Septation Initiation Network (SIN), which includes Cdc7p and the GTPase Spg1p, is emerging as a primary regulatory pathway used by fission yeast to control cytokinesis. A similar group of proteins comprise the Mitotic Exit Network (MEN) in budding yeast. This is the first direct evidence for the existence of a functional SIN-MEN pathway outside budding and fission yeast. In addition to SEPH, potential homologues were also identified in other fungi and plants but not in animal cells. Deletion of sepH resulted in a viable strain that failed to septate at any temperature. Interestingly, quantitative analysis of the actin cytoskeleton revealed that sepH is required for construction of the actin ring. Therefore, SEPH is distinct from its counterpart in fission yeast, in which SIN components operate downstream of actin ring formation and are necessary for ring contraction and later events of septation. We conclude that A. nidulans has components of a SIN-MEN pathway, one of which, SEPH, is required for early events during cytokinesis.