DNA segments cloned from Schizosaccharomyces pombe by the ability to complement Escherichia coli pyrB mutations are shown to complement a ural mutation in S. pombe, thereby demonstrating that ural is the structural gene for aspartate transcarbamylase of S. pombe. Further, such segments combined with parts or all of pBR322 are shown to be capable of autonomous propagation in S. pombe. This suggests the existence of an autonomously replicating sequence (ars) in the vicinity of ural. Unlike the TRP1 segment cloned from Saccharomyces cerevisiae [Struhl, K., Stinchcomb, D. T., Scherer, S. & Davis, R. W. (1979) Proc. Natl. Acad. Sci. USA 76, 1035-1039], plasmids carrying the ural locus do not multiply as monomers but assume a polymeric form as large as a decamer to an icosamer in the yeast. Monomers are tandemly arranged in the polymer. Inversion of an inserted fragment or insertion of another segment into a competent plasmid greatly decreases the efficiency of such transformation, implying a role of the tertiary structure of the plasmids in the establishment of transformation of this kind.