The African grass rat,Arvicanthis sp., from Benin shows sex‐chromosome polymorphism for the presence in the karyotype of three X‐chromosome forms: subtelocentric (Xst), submetacentric (Xsm), and metacentric (Xm). Here we report statistically significant differences of diploid genome size (g.s.) among individuals, from 6.22 ± 0.06 pg to 6.80 ± 0.10 pg, depending on the X‐chromosome form. The evaluation of the total and X‐chromosome heterochromatin content alone (Xst: 2.16 ± 0.22%; Xsm: 5.02 ± 0.22%; Xm: 6.54 ± 0.28%), supports this suggestion. DNA base‐pair preferential staining revealed that heterochromatin of both autosome and sex chromosomes is AT‐rich. Digestion of genomic DNA with sixteen restriction enzymes produced DNA bands representing the repetitive fraction of the genome. When these were used as probes for fluorescencein situhybridization (FISH), two hybridization patterns were found: a) FISH positive signals localized in the pericentromeric areas of every chromosome of the complement and b) chromosome specific signals co‐localizing with both the pericentromeric regions of every chromosome and the heterochromatic regions of Xsm and Xm, but not on the Xst chromosome. FISH performed with a (TTAGGG)n probe revealed a telomere‐only pattern.