1. an extrachromosomal, extramitochondrial DNA circle of about 3 μm length, occurring in S. pombe, was cloned in pBR322 and a detailed restriction map of the cloned sequence constructed. 2. Hybridization of this DNA with the ribosomal DNA from S. cerevisiae revealed regions of homology which were mapped in the cloned S. pombe DNA. 3. A transformation system was developed using the URA3 gene of S. cerevisiae and the 3 μm molecule. It was shown that the r-DNA part of the hybrid plasmid promotes replication in S. pombe. 