Transformation studies with Saccharomyces cerevisiae (bakers' yeast) have identified DNA sequences which permit extrachromosomal maintenance of recombinant DNA plasmids in transformed cells. It has been hypothesized that such sequences (called ARS for autonomously replicating sequence) serve as initiation sites for DNA replication in recombinant DNA plasmids and that they represent the normal sites for initiation of replication in yeast chromosomal DNA. We have constructed a novel plasmid called TRP1 R1 Circle which consists solely of 1,453 base pairs of yeast chromosomal DNA. TRP1 RI Circle contains both the TRP1 gene and a sequence called ARS1. This plasmid is found in 100 to 200 copies per cell and is relatively stable during both mitotic and meiotic cell cycles. Replication of TRP1 RI Circle requires the products of the same genes (CDC28, CDC4, CDC7, and CDC8) required for replication of chromosomaL DNA. Like chromosomal DNA, its replication does not occur in cells arrested in the B1 phase of the cell cycle by incubation with the yeast pheromone alpha-factor. In addition, TRP1 RI Circle DNA is organized into nucleosomes whose size and spacing are indistinguishable from that of bulk yeast chromatin. These results indicate that TRP1 RI Circle has the replicative and structural properties expected for an origin of replication from yeast chromosomal DNA. Thus, this plasmid is a suitable model for further studies of yeast DNA replication in both cells and cell-free extracts.