We have identified the cyclin domain-containing proteins encoded by the genomes of 17 species of  Aspergillus  as well as 15 members of other genera of filamentous ascomycetes. Phylogenetic analyses reveal that the cyclins fall into three groups, as in other eukaryotic phyla, and, more significantly, that they are remarkably conserved in these fungi. All 32 species examined, for example, have three group I cyclins, cyclins that are particularly important because they regulate the cell cycle, and these are highly conserved. Within the group I cyclins there are three distinct clades, and each fungus has a single member of each clade. These findings are in marked contrast to the yeasts  Saccharomyces cerevisiae, Schizosaccharomyces pombe , and  Candida albicans , which have more numerous group I cyclins. These results indicate that findings on cyclin function made with a model  Aspergillus  species, such as  A. nidulans , are likely to apply to other Aspergilli and be informative for a broad range of filamentous ascomycetes. In this regard, we note that the functions of only one  Aspergillus  group I cyclin have been analysed (NimE Cyclin B  of  A. nidulans ). We have consequently carried out an analysis of the members of the other two clades using  A. nidulans  as our model. We have found that one of these cyclins, PucA, is essential, but deletion of PucA in a strain carrying a deletion of CdhA, an activator of the anaphase promoting complex/cyclosome (APC/C), is not lethal. These data, coupled with data from heterokaryon rescue experiments, indicate that PucA is an essential G 1 /S cyclin that is required for the inactivation of the APC/C-CdhA, which, in turn, allows the initiation of the S phase of the cell cycle. Our data also reveal that PucA has additional, non-essential, roles in the cell cycle in interphase. The  A. nidulans  member of the third clade (AN2137) has not previously been named or analyzed. We designate this gene  clbA . ClbA localizes to kinetochores from mid G 2  until just prior to chromosomal condensation. Deletion of  clbA  does not affect viability. However, by using a regulatable promoter system new to  Aspergillus , we have found that expression of a version of ClbA in which the destruction box sequences have been removed is lethal and causes a mitotic arrest and a high frequency of non-disjunction. Thus, although ClbA is not essential, its timely destruction is essential for viability, chromosomal disjunction, and successful completion of mitosis.