The Venezuelan vole mouse, Akodon urichi venezuelensis, is a small South American cricetid which can be reared in the laboratory. Their somatic chromosomes have been morphologically and statistically investigated. The chromosome number is 2n = 18. The eight pairs of autosomes are metacentric; four pairs are similar in size and arm ratio, but the others are easy to identify. In males there is an XY sex-chromosome pair; the X is a submetacentric and duplicate type. Polymorphism has been found in the small-sized Y chromosome; in two out of six individuals it was an acrocentric and in the remaining four, a fully telocentric chromosome. In the females, the two X chromosomes are slightly heteromorphic; there is an X! quite similar to the male X chromosome and a longer X2, which differs also from the X1 in arm ratio. The risk of reversals of order in similar chromosomes is discussed.