A mutant with unstable maintenance of hybrid plasmids containing either one of the centromeric loci CEN3, CEN6, CEN11 and ars1 or the replicator of the 2 mu plasmid has been obtained. The frequency of loss of hybrid plasmids in the mutant was up to 3.10(-1) per one generation versus 10(-2) in the original strain. The unstable maintenance of minichromosomes in the mutant is controlled by a recessive nuclear gene, named SMC for stability of minichromosomes. Loss of some minichromosomes is connected with impairment of their segregation in cell division. In diploids homozygous for smc mitotic chromosomal segregation is not affected but sporulation is impaired. The question of adequacy of usage of minichromosomes for selection of mutants with impaired function of centromeric loci is discussed.