AbstractIndividually reared mother‐infant dyads of crab‐eating monkeys (Macaca fascicularis) were observed cross‐sectionally in their mother‐infant relationship. In infants aged from 0 to 5.0 months, rather drastic changes were found both at 0.5–1.0 and 2.0–3.0 months of age. For an explanation of these changes, developmental processes of discrimination between mother and infant were analyzed cross‐sectionally by exchanging mother‐infant combinations. The results indicated that the first 0.5 months postpartum was characterized as a behaviorally nondiscriminating stage where nipple discrimination by infants was the only exception. The latter half of the first month was the beginning of a nonaggressive discrimination stage by mothers, indicated by lipsmacking and sniffing and in infants by clinging. When infants reached the age of 2.0–3.0 months, the mothers' nonaggressive discimination with lipsmacking decreased, and her aggressive discrimination of alien infants increased. In addition to maternal visual discrimination of infants' physical appearances, differences in infants' odor and/or their method of nipple contact were suggested to affect the mothers' diffe