Aspergillus fumigatus produces conidia that are highly dispersable and resistant to degradation. We have sought to analyze these properties by studying the rodlets which form the outer spore coat protein. Degenerate primers based on hydrophobins in other fungi were applied to genomic DNA from A. fumigatus in PCR. A product of this reaction with similarity to an Aspergillus nidulans gene as judged by Southern hybridization was chosen for further study. Cloning and sequencing revealed a gene with two introns which encodes a protein of 159 amino acids. Structural characteristics consistent with those of other fungal hydrophobin genes, especially conserved cysteine residues, are present. The expression of the gene is limited to the developmental stages in which maturing conidiophores are present. This A. fumigatus gene, HYP1, was used to transform a mutant strain of A. nidulans that lacks rodlets. Transformants with a single copy of HYP1 expressed a rodlet layer on their conidia as observed by freeze-fracture electron microscopy.