A small protein with a molecular mass of 15 kDa, which is specifically found on the hyphal surface of a (+) mating-type strain of the model zygomycete Absidia glauca, was purified to electrophoretic homogeneity and partially sequenced. The corresponding gene was cloned by means of an oligonucleotide probe deduced from the protein sequence. It could be localized on an extrachromosomal circular DNA element with a total length of 1250 bp. Electron microscopic analysis of A. glauca DNA showed that small extrachromosomal DNAs with varying length are a common feature of this zygomycete. There are no indications of additional chromosomal copies of the gene for this surface protein, and the plasmid is absent from DNA preparations of the (-) mating type. The copy number ranges around three per haploid genome, and a single transcript with a length of 400 bp, coding for the surface protein, could be found by employing a hybridization probe which spans the complete fungal plasmid. This is the first report of naturally occurring extrachromosomal DNA in a Mucor-like fungus, and the only example where an integral protein of the cell wall is encoded by a plasmid.