A maternal infanticide was observed in a group of unprovisioned wild black-fronted titi monkeys (Callicebus nigrifrons). An approximately 3-day-old male infant was killed by his mother. A post-mortem revealed the infant to be clinically healthy. We considered various hypotheses to explain why this behavior occurred (e.g., reproductive advantage, stress, nutritional, infant viability and population density). It is noteworthy that the mother and not the father killed the infant, since in this species the father provides considerable infant care from a few hours after birth.