The possibility of cloning filamentous fungal genes by expression in the yeast Saccharomyces cerevisiae has been studied. A genome bank of Aspergillus niger was made in E. coli using a yeast cosmid shuttle vector and over 10,000 different cosmid clones were individually isolated. Yeast transformants carrying Aspergillus DNA were screened for the expression of the genes for fungal secreted glycoproteins, β-galactosidase, β-glucosidase, and amyloglucosidase, and for the expression of fungal genes complementing yeast ura3 and leu2 mutations. Of the five Aspergillus genes studied, only one, β-glucosidase, was found to be expressed in yeast, and this at a low level. This suggests that there are essential differences between the genes of yeast and filamentous fungi.