The formation of a number concept of threeness was studied in rhesus monkeys; which were required to discriminate on the basis of number differences between geometric forms mounted on poster board cards. The animals were tested in both two-stimulus and three-stimulus discrimination situations. Extraneous cues, such as size or stimulus arrangements, which might form a basis for the solution of the problems, were controlled and the effect of introducing new colors and new forms was measured. The monkeys learned a number of concepts of threeness at a moderately proficient level. The results are compared with other studies of capacity for number learning in animals.