The chromosomes of the two closely related species Sorex araneus and S. gemellus were investigated by various methods for the determination of constitutive heterochromatin (C-band techniques, treatment of cell cultures with AT- and GC-base-pair-specific compounds, and analysis of late-replication patterns). With the exception of the Yi sex chromosomes, no distinct heterochromatic regions like those in other mammals were demonstrated. Although use of a modified C-band technique produced moderately darkly stained bands, these did not have a definite heterochromatic character. Spontaneous sister chromatid exchanges (SCE’s) occurred significantly more frequently in the Yi chromosomes than in other chromosomes. Analysis of intrachromosomal frequencies of SCE’s showed no significant deviations from a uniform random distribution.