A Robertsonian karyotypic polymorphism in the common shrew in the Oxford area, first described in the 1950s, was re-examined. The polymorphism involves chromosome arm combinations kq, no and pr (characteristic of the Oxford karyotypic race), ko (characteristic of the Hermitage karyotypic race) and jl (found in both races). The polymorphism for jl was sporadic along a north-south transect through the Oxford area, with the frequency of the twin-acrocentric morph never exceeding 10%. The frequency of the Oxford race-specific metacentrics decreased and the frequency of the Hermitage race-specific metacentric ko increased from north to south along the transect. At a latitudinal grid reference of about 180 km, there was a high frequency of individuals with chromosome arms k, n, o and q in the ancestral acrocentric state. This was coincident with the area of occurrence of ko-kq and ko-no Oxford-Hermitage hybrids. Such hybrids are double Robertsonian heterozygotes with monobrachial homology and are likely to suffer reduced fertility in consequence. It is proposed that this is a source of selection against the monobrachial hybrids and hence results in an increase in frequency of the acrocentric morphs. This scheme goes some way to explain the clines of polymorphism for arm combinations kq, no and ko, but it is suggested that other selective factors are involved. It cannot explain the cline of polymorphism for pr, which is in general terms similar to that for kq and no, but is more shallow and centred further north.