The karyotype of the isopod crustacean Asellus aquaticus does not normally display any heteromorphic sex chromosome pair. Some of the males in a wild population of A. aquaticus collected in the Sarno river near Naples do display a heteromorphic chromosome pair. The hetero-morphism is due to the presence of two intercalary heterochromatic areas on one chromosome. This chromosome is inherited through the male line as a normal Y chromosome. The heteromorphic pair has retained the capacity to recombine during meiosis. In-situ hybridization of ribosomal probes, labelled with digoxigenin-dUTP, reveals that ribosomal sequences are associated with both intercalary heterochromatic areas of the heterochromosome. The ribosomal genes are normally telomeric and associated with heterochromatin. After digestion of genomic DNA with BamHI EcoRI and HindIII restriction endonucleases and hybridization with ribosomal probes, the hybridization patterns of the males with the heterochromosome differ from those of the males without the heterochromosome, as well as from those of the females, which are identical. The possible origin of the morphological differentiation of the heterochromosome, and the causes of the differences in the ribosomal DNA restriction patterns linked to the presence of this chromosome, are discussed.