Sixty-eight cranial dimensions of the St Kitts green monkey have been compared with the corresponding ones in the African green monkey. The skulls of the island monkeys are now bigger and less variable than those of the modern mainland descendants of its parent stock. The most meaningful changes have occurred in measurements of the breadth of the skull. No changes in the variability of meristic cranial characters have been found. The changes in the dimensions of the skull parallel those that have occurred in the teeth. It is possible that both result from the action of natural selection on a third reacting system, perhaps general body-size.