Eight independent chl (chromosome loss) mutants were isolated using yeast haploid strain disomic for chromosome III. In these mutants, chromosome III is lost during mitosis 50-fold more frequently than in the wild-type strains. chl mutants are also incapable of stable maintenance of circular and linear artificial chromosomes. Seven of the eight mutations are recessive, and one is semidominant. Complementation tests placed these mutants into six complementation groups (chl11 through chl16). Based on tetrad analysis, chl12, chl14 and chl15 correspond to mutations in single nuclear genes. Tetrad analysis of the other mutants was not possible due to poor spore viability. Complementation analysis was also carried out between collection of chl mutants and ctf mutants (chromosome transmission fidelity) (Spencer et al., 1990). The chl3, chl4, chl8, chl12 and chl15 mutants were unable to complement ctf3, ctf17, ctf12, ctf18 and ctf4, respectively. Three CHL genes were mapped by tetrad analysis. The CHL3 gene is placed on the right arm of chromosome XII, between the ILV5 (33.3 cM) and URA4 (21.8 cM) loci. The CHL10 gene is located on the left arm of chromosome VI, 12.5 cM from the centromere. The CHL15 gene is tightly linked to the KAR3 marker of the right arm of chromosome XVI (8.8 cM). The mapping data indicate that these three genes differ from other genes known to affect chromosome stability in mitosis. Therefore, the total number of the CHL genes identified (including those described by us earlier) is 13 (CHL1-CHL10, CHL12, CHL14 and CHL15).