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Papilio gothica (Papilionidae) is described as a new species from montane regions of Colorado and adjacent states. It is a sibling of the lowland Californian P. zelicaon and is characterized by subtle color and pattern differences, univoltinism, Pseudocymopterus montanus (Umbelliferae) as larval food, and probably distinctive flight behavior. The chromosome number is n = 30. Phenotypically P. gothica and P. zelicaon are nearly alike, but their F1 hybrids with P. polyxenes are unlike. They have different hybrid sex ratios in their crosses with P. polyxenes, gothica × polyxenes being nearly lethal for the sex of the polyxenes parent whereas zelicaon × polyxenes has more nearly equal sex ratio although deficient in the sex of the polyxenes parent. The evolutionary meaning of the principle of phenotypic stability with phyletic divergence is discussed. Phenotypic, biological, and geographic characters are summarized for the North American taxa similar to gothica, especially P. brucei, P. oregonia, P. rudkini, and P. hudsonianus. Photographs are given for all of these as well as P. gothica and P. zelicaon and hybrids of both crossed with P. polyxenes and P. bairdii. The foodplant of P. hudsonianus in Manitoba is Zizia (probably aurea) (Umbelliferae). Egg hatchability and embryonic viability are tabulated for P. gothica, P. zelicaon, and several hybrid combinations.