Out of 72 fox skulls (Vulpes vulpes L.) collected in the North Holland Dune Reserve (NHD) between 1979 and 1985, 12 (16.7%) skulls showed a pronounced protrusion of the maxillary incisors over the mandibular incisors. Comparison of dimensions of affected and normal skulls revealed that this protrusion was the effect of a shortening of the front part of the mandibles. The mode of inheritance of this aberration is thought to be under recessive monogenetic control. This was deduced from the family relations between individual foxes radio-collared or earmarked during five years of ecological research, and from the clear bimodal distribution in mandible length. The high incidence of the aberration can be explained by the history of the fox population in the NHD. Before 1968 the NHD was not inhabited by foxes as a result of its ecological isolation. It is rumoured that in that year four cubs, probably from one litter, were set free. It is believed, therefore, that the present population has originated from a small and isolated gene pool. Compared to normal foxes, affected animals are likely to be at an ecological disadvantage. Hence it is expected that the incidence of the aberration will decline in the future, since the population density reached a stable maximum around 1982 and the selective forces against the aberration will presumably be stronger now than during the phase of rapid population growth.