The method used to estimate the total population of the Skomer vole (Clethrionomys glareolus skomerensis) on Skomer Island in September 1960 is described. The technique involved both line and grid trapping and the deductions to be made from these methods are discussed. In addition to the vole large numbers of the field mouse (Apodemus sylvaticus) were trapped and information was gathered on the ecological relation between the two species. The vole was most numerous in dry, deep bracken but also occurred in smaller numbers in the areas having scattered bracken with grass undercover. Field mice were trapped in greatest numbers in the more exposed areas, especially on eliff slopes and on bare rock outcrops. The distribution on Skomer Island of these two species is discussed. Figures for the population sizes of both voles and field mice on two areas are given. Half the voles and field mice captured were young animals. Breeding was still in progress. Data are presented for body weights, in relation to sex and age, for both species.