A Polybia accidentalis wasp was conditioned to collect sugar-water from a blue card (CB). In discrimination experiments, the wasp was able to detect the CB card when it was placed among 11 other cards that were all white, different shades of gray, different colors, or different shades of blue. The wasp made fewer mistakes in later trials than in earlier trials of each experiment, showing that she learned to focus on cues that were sufficient for discriminating between the CB card and the other cards present. The wasp was very successful ai discriminating the CB card from other cards of similar brightness but that differed in their spectral reflection properties. The wasp's most common mistakes were with cards that were most similar to the CB card in their relative spectral reflection curves at wavelengths shorter than 575 nm Cards that were similar in this region, and also reflected longer wavelengths, were most commonly confused with the CB card. Cards with reflection peaks in the blue region but that also reflected other wavelengths shorter than 575 nm were not commonly confused with the CB card. Therefore, unlike a few hymenopteran species. Polybis does not seem to have a red-sensitive photoreceptor. Color vision was supported in that the wasp discriminated between the CB card and cards that varned in their spectral reflection properties, independent of light intensity. This is the first behavioral demonstration of color vision in the Polistinae and could further our understanding of the evolution and ecology of this Urge group.