During a long-term field study on the behavioral ecology and communication of a population of the monogamous Milne-Edwards' sportive lemur (Lepilemur edwardsi) in NW-Madagascar, we recorded for the first time an infanticide in a nocturnal lemur. A male newcomer killed the infant of a female whose male partner had left her recently. Both the social-pathology and the sexual-selection hypotheses may explain infanticidal behavior in Lepilemur edwardsi.