Abstract.1. The tiger swallowtail butterfly, Papilio glaucus (L.) (Lepidoptera: Papilionidae), is commonly assumed to have a bivoltine life cycle in central New York State and other parts of the northeastern United States. The seasonal abundance of adults in Tompkins County, New York, shows a bimodal pattern in some years, and a skewed unimodal pattern in others. We studied the incidence of diapause, the potential for successful development of two generations in the field, and the dates of adult emergence from overwintering pupae to determine whether these seasonal patterns are the result of bivoltinism.2. Insects from Tompkins County, reared as larvae under 16L:8D photoperiod, uniformly enter pupal diapause.3. Comparison of the heat units required for successful development of two generations with the heat units available in the field during 11 years shows that emergence of non‐diapausing individuals would occur too late in the summer to contribute to the second peak of butterfly abundance. Offspring of non‐diapausing individuals could only rarely complete development before the end of the growth season.4. Diapausing offspring of butterflies collected in early June and late July differed sufficiently in dates of emergence to account for the field pattern of seasonal abundance.5. Electrophoretic studies revealed no evidence of genetic isolation between early and late emerging butterflies.6. These results suggest that polymodal emergence of diapausing insects can masquerade as a bivoltine life cy