The Polo-like kinases (Plks) are conserved, multi-functional cell cycle regulators that play additional roles in metazoan development. We previously identified plkA in Aspergillus nidulans, the only Plk investigated in filamentous fungi to date, and partially characterized its function through overexpression. We now report the plkA null phenotype. Surprisingly, plkA was not essential, unlike other fungal Plks. A subset of )plkA cells contained defects in spindle and chromosome organization, supporting some conservation in cell cycle function. However, septa were present, suggesting that PLKA is not a central regulator of septation like other Plks. The )plkA colonies were compact with multi-branched hyphae, implying a novel role for PLKA in hyphal morphogenesis. These defects were suppressed by high temperature or low benomyl concentrations, suggesting that PLKA functions in part through influencing microtubule dynamics. However, )plkA colonies also demonstrated benomyl and temperature-insensitive decreases in conidiation and precocious formation of Hulle cells. This represents the first example of a link between a Plk and development in fungi, and suggests that PLKA negatively regulates sexual reproduction through distinct mechanisms. Phylogenetic analyses suggest that PLKA and filamentous fungal Plks are related to the divergent metazoan PLK4, whereas yeast Plks group with metazoan PLK1-3. Thus, PLKA has some conserved functions, but may play additional novel roles in influencing morphogenesis and negatively regulating sexual development.