The extent and character of dimensional differences between the teeth of the African and St Kitts green monkeys have been compared with corresponding differences between the African green on the one hand and its two fellow subspecies, the vervet and grivet, and its two fellow species, the white-nosed monkey and moustached monkey, on the other. Dimensions which tests showed to be highly correlated were grouped, and the means of the groups compared; dental dimensions were also analyzed by means of discriminant functions. The extent to which the permanent teeth of the St Kitts monkey now differ from the modern African descendant of its parent species is greater than, but in a direction opposite from, the difference which separates the white-nosed monkey from the African green. It is also approximately two-thirds the difference which separates the vervet from the African green, but considerably less than the difference which separates the grivet and moustached monkeys. The comparisons also showed that different teeth differed in the extent to which they diverged from those of the African green. The greatest divergence in the St Kitts, the white-nosed and the moustached monkeys was observed in the lateral dimensions of the canines; and in the vervet and grivet in the transverse dimensions of the incisors. The speed at which the changes in the St Kitts monkey have occurred appears to be from 5000 to 10,000 times the estimated rate of evolution in the teeth of horses over a period of from 5 to 16 million years.