One of the causes of nest failure in woodpeckers is competition for nest sites among cavity-nesting bird species, expressed through aggressive interactions. Here I report a case of mortal aggression by an adult White-fronted Woodpecker (Melanerpes cactorum) toward nestlings of the White-barred Piculet (Picumnus cirratus), presumably caused by interference competition, in the dry Chaco woodlands of northwestern Argentina. The woodpecker killed 2 piculet nestlings but did not consume or remove them to feed its own nestlings. The attack could have been motivated by territorial defense, and to reduce competition for nest sites, since the piculet nest was within the territory of the woodpecker, and near its active nest and one of its most consumed sap trees. The observations described here provide new ethological information about the levels of aggression of the White-fronted Woodpecker, and suggests that interspecific competition likely could constitute one of the causes of nest failure among Neotropical woodpeckers.