In the fission yeast, Schizosaccharomyces pombe, transformation with recombinant plasmids always results in a high proportion of mitotically unstable transformants. This suggested that specialised (ARS) sequences might not be required for autonomous replication of plasmids in S. pombe, contrary to the situation in Saccharomyces cerevisiae. We have shown that specialised ARS sequences, analogous to those in S. cerevisiae, do exist in S. pombe, supporting the view that ARS elements are a general feature of eukaryotes. In addition, there is a further mechanism of plasmid maintenance which involves homologous and non-homologous integration into, and excision from the genome.