The fungi are one of the most diverse groups of organisms within terrestrial ecosystems. Although they share many morphological features in common with plants, phylogenetic studies have shown that they represent a distinct kingdom with similarities to both plants and animals. The pronounced impact of fungi on human welfare can be felt in many ways. As prominent pathogens of plants and animals, they are responsible for considerable economic hardship and loss of life. On the other hand, fungi have traditionally been manipulated by humans for the production of important foodstuffs such as bread and beverages. Finally, in the laboratory, fungi have served as powerful model organisms for the characterization of basic cellular processes. For example, in the 1940's, Beadle and Tatum used the filamentous fungus Neurospora crassa to demonstrate the principle of one gene-one enzyme. More recently, the first eukaryotic genome to be completely sequenced was that of the yeast Saccharomyces cerevisiae. We use the filamentous fungus Aspergillus nidulans to elucidate the mechanisms underlying cellular morphogenesis. Our goal is the exploit the simple genetics of this fungus to determine how morphogenesis is regulated in both space and time.