We describe here recent work on the molecular genetics of mitosis in the filamentous fungus Aspergillus nidulans. Aspergillus is one of three simple eukaryotes with powerful genetic systems that have been used to analyze mitosis. The modern molecular biological techniques available with this organism have made it possible to use mutations to identify genes and proteins that play an important role in mitosis. Three Aspergillus genes that affect mitosis are described. One gene, nimA, is specifically expressed late in the cell (...) cycle and codes for a putative protein kinase that induces mitosis, even in cells blocked in S‐phase. The second gene, bimG, codes for a putative phosphatase that interacts functionally with the nimA kinase. The third gene, bimE, codes for a protein that suppresses mitosis during interphase, apparently by keeping nimA turned off. None of these genes appear to be similar to any of the genes affecting mitosis that have been characterized in other eukaryotes, but rather appear to be elements of a system that prevents mitosis from occurring during interphase. (shrink)