The genetic stability of wheat/rye ('Chinese Spring'/'Imperial') disomic addition lines was checked using the Feulgen method and fluorescent in situ hybridization (FISH). Feulgen staining detected varying proportions of disomic, monosomic, and telosomic plants among the progenies of the disomic addition lines. The greatest stability was observed for the 7R addition line, while the most unstable lines were those with 2R and 4R additions. Chromosome rearrangements were also detected using FISH. Based on the specific hybridization patterns of repetitive DNA probes pSc119.2 and (AAC)(5), as well as ribosomal DNA probes (5S and 45S), isochromosomes were identified in the progenies of 1R and 4R addition lines. The results draw attention to the importance of continuous cytological checks on basic genetic materials by using FISH, because this method reveals chromosome rearrangements that could not be detected either with the conventional Feulgen staining technique or with molecular markers.