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Summary Interspecific facultative social parasitism is well known in ants and in bumble-bees, but it is rarer in wasps. This form of parasitism is traditionally considered to be an intermediate stage in the evolution of obligate interspecific parasitism, where the parasites are no longer able to nest alone. We report field and experimental observations of a newly discovered facultative parasitic relationship between two closely related free-living Polistes species: P. nimphus and P. dominulus. P. nimphus foundresses sometimes usurp the nests of the larger P. dominulus before worker emergence. The invading queen takes over the nest with abundant abdomen stroking on the nest surface and is accepted by workers if they emerge 6 or more days after usurpation. Morphometric comparisons show that the usurper species, though smaller than its victims, has morphological adaptations consisting of larger heads, mandibles and front femora relative to their body size that may give it an advantage during nest invasion. This strategy is likely to be taken only after the foundress loses her original nest because invading P. nimphus queens have lower reproductive success than they would have had on their own nest. Overall, we found that P. nimphus usurpers use strategies of invasion similar to those of two obligate parasites, suggesting that this may be an example of one of the pathways by which social parasitism evolved.