The cytological instability of common wheat-rye addition lines was investigated in the present study. The chromosome numbers of almost all addition lines were considerably stable, but those of CS + 5R were very variable. The rye chromosome added in this line was found to be much shorter than expected. Fluorescent in situ hybridization with 5S rDNA and the centromere-specific probes clearly revealed that the short rye chromosome contains only a short arm of chromosome 5R (5RS). In this line, chromosome numbers of both 5RS and common wheat were changeable. The chromosome numbers ranged from 2n = 36 to 2n = 44 in the cells carrying two 5RS, and ranged from 2n = 31 to 2n = 44 in one 5RS cells. In addition to the chromosome instability, the multicells wrapped in a sac-like structure were frequently observed in the root meristematic tissues of CS + 5RS after the enzyme treatment for chromosome preparation. Genomic in situ hybridization with rye DNA as a probe showed that all cells in sacs investigated were at the interphase stage and contained one or two 5RS chromosomes. An electron microscopic analysis revealed that the cells of CS + 5RS, particularly in sacs, have abnormal (irregular and curved) cell walls. These results indicate that 5RS has (a) specific factor(s) influencing the cell wall development as well as the genome stability. The cytological instability of common wheat-rye addition lines was investigated in the present study. The chromosome numbers of almost all addition lines were stable stable, but those of CS + 5R were very variable. The rye chromosome added in this line was found to be much shorter than expected. Fluorescent in situ hybridization with 5S rDNA and the centromere-specific probes clearly revealed that the short rye chromosome contains only a short arm of chromosome 5R (5RS). In this line, chromosome numbers of both 5RS and common wheat were The chromosome numbers ranged from 2n = 36 to 2n = 44 in the cells carrying two 5RS, and ranged from 2n = 31 to 2n = 44 in one 5RS cells. In addition to the chromosome instability, the multicells wrapped in a sac- like structure were frequently observed in the root meristematic tissues of CS + 5RS after the enzyme treatment for chromosome preparation. Genomic in situ hybridization with rye DNA as a probe showed that all cells in sacs in vestigated were at the interphase stage and contained one or two 5RS chromosomes. An electron microscopic analysis revealed that the cells of CS + 5RS, particularly in sacs, have abnormal (irregular and curved) cell walls. These results indicate that 5RS has (a) specific factor (s) influencing the cell wall development as well as the genome stability.