A study of the variation in infantphysical development was undertaken in a colony of captive rhesus macaques (Macaca mulatta). Somatic growth and dental development in 32 infants born in 1987 and 1988 were regularly monitored between 1987 and 1990. Physical characteristics of the mothers of each infant and other adult females in the colony were also studied over the same period. The variation seen in somatic growth and dental development are outlined in detail describing the different patterns of development that were shown by the infants. Two distinct patterns of growth emerge, with a number of infants showing a pre-pubertal growth spurt and maintaining their physical advantage over the others. There is very little relationship between dental development and post-cranial somatic growth. Dental development together with cranial growth appear conservative and seem to progress on a set course not related to other aspects of somatic growth. The role of the parents, the social group and mother-infant behaviour are examined in detail. The size of the mother is strongly related to infant growth, and her age and maturity also have an effect. The dominance rank of the mother is related to patterns of pre- and early post-natal investment. Infant suckling and feeding behaviour are related to cranial development in males, and the size of the infant in relation to the mother may be important in the timing of weaning in daughters. A high degree of variation in neonate size was seen between the different social groups which may relate to paternity. There is evidence to suggest that life history events and social stress may be reflected in microscopic dental development in young rhesus monkeys. This work has outlined how important the mother and the environment are in studies of growth and maternal investment in non-human primates. It has also provided details relating to intra-specific variation in development that can be examined in the light of current life history theory. Implications for the study of fossil hominids are discussed.