Much of what is known about development in cercopithecine primates is derived from studies of baboons Papio sp and macaques Macaca sp. Two closely spaced births at Cleveland Metroparks Zoo enabled us to describe the development of single infants representing two little-studied species: the Wolf's guenon Cercopithecus pogonias wolfi and Allen's swamp monkey Allenopithecus nigroviridis. Chance provided controls for variables known to affect mother–infant relations including environmental quality, maternal parity and experience, and infant gender. We examined the development of infant behaviour and spatial independence over the first year of life. The infants exhibited developmental trends similar to those reported for other guenons. Both infants were largely capable of feeding themselves by their third month. Both infants gradually spent more time away from their mothers as they aged and both played a dominant role in regulating proximity relationships with their mothers from their first month. Several proximity measures indicate that the Allen's swamp monkey attained spatial independence more rapidly than the Wolf's guenon, a result that could be owing to the higher levels of aggression within the Wolf's guenon group. In addition to informing husbandry practices for guenon breeding groups, these data demonstrate the power of zoos for contributing basic information of this nature.