I observed a population of White-winged Trumpeters (Psophia leucoptera) in undisturbed lowland moist forest in Peru. During 2,460 h of focal observations, I observed one instance of infanticide that occurred when a permanent group killed the 7-day-old chick of an intruding group that was using a portion of its territory. Trumpeter groups defend large, year round, contiguous territories that barely provide groups with enough food to meet their caloric requirements during the dry season when fruit abundance is low. There may be a selective advantage to such infanticidal behavior if it helps prevent the formation of a new group on a resident group's territory, and the concomitant loss of space that would accompany such an event.