Core principles of ecological theory predict that, in the absence of other factors, mutualisms should experience destabilizing positive feedback and should be vulnerable to extinction through competitive exclusion by exploiter species. Many effective stabilizing mechanisms address one issue or the other, and many turn upon additional features. Using an explicitly demographic approach, I show that indirect, demography-mediated interactions between mutualists and exploiters can enable mutualist-exploiter coexistence, which in turn can stabilize the abundances of mutualists, exploiters, and their shared resources. This occurs because of the distinct resource demographic responses that are inherent to interaction with mutualistic and exploitative partners and can occur in long-lasting, exclusive interactions, such as protection mutualisms, as well as in apparently very different, short-lived mutualistic interactions, such as pollination. The key necessary factor-demographic response to interspecific interaction-is common in nature. Some demographic structure is also necessary and is generated through interspecific interaction in long-lasting associations; it is also very common in natural populations. Thus, the explicitly demographic and multispecies approach taken here constitutes a potentially promising single explanation for the apparent stability of mutualism in a wide range of natural systems.