Electrophysiological and behavioural methods have been applied to 34 species of the primates and, for comparison, to the Madagascan hedgehog to determine their responses to the proteins thaumatin and monellin. These substances elicit an intensely sweet taste sensation in man. All Catarrhina prefer monellin to water. The responses of the Prosimii as well as those of the South American primates to monellin are different, some species show a reaction, other species are not sensitive. In the case of thaumatin neither the Prosimii – including Tupaia and Tarsius – nor the South American primates show any response to this protein. Only the Cercopithecidae, the Hylobatidae and the Pongidae respond to this protein like man and prefer this substance to water. This physiological aspect of taste constitutes a clear dichotomy within the order Primates. This capability to taste thaumatin probably developed as long as 38 million years