While some primates attempt to avoid predation by fleeing or hiding, others actually approach, harass, and sometimes attack potential predators, a behavior known as mobbing. Mobbing seems to invite obvious risks, but predators have rarely been observed to injure or kill animals that harass them. The true costs of predator mobbing and the functional significance of this behavior remain poorly understood. Here, we report a fatal attack by a jaguar (Panthera onca) on a mobbing capuchin (Cebus capucinus) that we observed on Barro Colorado Island, Panama. This rare observation illustrates that mobbing predators such as large felids can, indeed, be costly. However, to understand fully the costs of mobbing, the energetic and opportunity costs that primates incur when they harass predators must also be considered.