A genomic DNA library was constructed from a pathogenic strain of Aspergillus fumigatus using the cosmid vector pCosAX. Cosmid clones with homologies to the rodA, brlA, fluG, flbA or trpC genes from A. nidulans were isolated from the library. Each A. fumigatus clone was used to complement a strain of A. nidulans with a mutation in the homologous gene. A spontaneous white spored strain of A. fumigatus was isolated. The mutation was complemented by transforming the strain with a plasmid containing the wA gene from A. nidulans. The results from these experiments indicate a significant degree of structural and functional homology between genes from the organisms. These findings indicate the potential to exploit the methods and information available from A. nidulans to address questions related to human disease caused by A. fumigatus and the ability to use A. nidulans as a surrogate genetic host for characterizing A. fumigatus gene function.