It can be shown theoretically that when the benefits of mutualism are shared among a group of neighbors, then the evolution of mutualism is impeded. This statement, however, is not as insightful as it might seem, because so little is known about the values of important parameters such as the costs and benefits of mutualism, or salient aspects of population structure. This study shows that a species of phoretic mite (Poecilochirus necrophori: Acari, Parasitidae) strongly enhances the breeding success of its beetle carrier, even though the benefits of this mutualism must be shared among a fairly large group. Thus, the group size argument should not be used to argue categorically against the prevalence of mutualism in nature.