A transformation system has been developed for Mucor circinelloides, by direct cloning of a wild-type methionine gene that complements the auxotrophic mutation. The marker gene isolated was associated with an autonomous replication sequence (ARS) functional in this zygomycete. Southern hybridisation analyses of transformants showed sequence homology both with vector DNA and with Mucor wild-type DNA. The transformation frequency (up to 6000 per microgram DNA) and the mitotic instability of the transformed cells were studied. The hybridisation pattern of undigested DNA from the transformants suggests that the inserts contain a novel autonomous replication element for this filamentous fungus.