Filamentous fungi, and in particular Aspergillus nidulans and Neurospora crassa, have been extensively used as models for eukaryotic gene organization and gene regulation. These organisms are very amenable to such studies, and the methodology for classical genetic analysis is highly sophisticated, making use of sexual and parasexual crosses, as well as heterokaryotic and diploid states. The extreme metabolic versatility of filamentous fungi and their characteristic cell differentiation provide areas of study not possible in Saccharomyces cerevisiae.