Intact ungerminated spores or germinating swollen spores of Mucor circinelloides were biolistically transformed to leucine prototrophy with the autonomous replicating plasmids pLeu4 and pMCL1302. The former developmental stage was efficiently transformed with both plasmids. The existence of pLeu4 sequences, as selfreplicating elements, within Leu+ transformants was established by Southern analysis and by the recovery of plasmid pLeu4 in bacteria transformed with total DNA from M. circinelloides transformants. This is the first report of the transformation of intact cells of a member of the Zygomycetes.