YEp vectors carrying the GPD promoter and PGK terminator for constitutive expression showed high partition efficiencies specifically in [cir(0)] strains, when DNA fragments were inserted into the cloning site. These plasmids appeared to be more stable than yeast centromeric plasmids, being lost in less than 10(-2) cells after each generation, and more than 90% of the cells carried the plasmids after 20 generations of growth in the absence of selective pressure. The REP3 region was essential together with the GPD promoter and PGK terminator for plasmid equipartitioning in [cir(0)] strains. The present results suggest that this host-vector system would be useful for astute observation of the phenotype caused by gene overexpression, and for heterogenous protein production using natural medium, because of efficient partitioning of the plasmid without selective pressure.