Müllerian mimicry, where 2 or more unrelated aposematic species resemble one another, is predicted to reduce the per capita mortality of co-mimics by allowing them to share the cost of educating naïve predators about their unpalatability. However, the specific assumptions and predictions of Müller's theory of shared resemblance have been previously unsupported; some authors have suggested that the benefits of signal similarity are undetectable or at best very small. We demonstrate clearly and unambiguously that mimicry between 2 defended forms can provide substantial protection from uneducated predators in the manner proposed originally by Müller. By utilizing prey signals that were designed and demonstrated, to be equally visible, learned with equal facility, and discriminated by our predators, we assessed the effect of the presence of signal mimicry on the survival of a Model species in a "novel world" experiment, with wild-caught great tits (Parus major) as predators. We found that the net effect of mimicry was mutualistic, with co-mimics showing increased survivorship through shared predator learning. Visually distinct prey showed a mortality benefit from coexistence even without signal mimicry as a result of a density-dependent dilution effect. Perfect mimicry provided an added benefit of enhanced predator avoidance learning, and our results suggest that the benefits of shared warning signals may be even stronger than Müller originally proposed. Copyright 2010, Oxford University Press.