The Cosmopterigid moth genus Hyposmocoma may rival or exceed Drosophila in terms of endemic Hawaiian species diversity. The current number of recognized species is already near 400, though comparatively little research has been conducted. As one of the greatest radiations in the Hawaiian Islands, Hyposmocoma could provide a powerful comparison to Hawaiian Drosophila with respect to patterns of diversification and evolutionary constraints in the broader context of evolutionary theory. Additionally, patterns of diversity in Hyposmocoma may serve as a source of information for the prioritization of regions and habitats on which conservation attention should be focused, and the scale at which conservation might be most effective. I present a systematic hypothesis, based on sequence from one mitochondrial and two nuclear genes, for an unusual group of Hyposmocoma, which has evolved a semi-aquatic lifestyle. Results suggest multiple invasions and reversions of the aquatic habit-which may be an extremely rare evolutionary scenario due to the constraints of an aquatic lifestyle. Aquatic Lepidoptera are a global evolutionary anomaly, making Hawaii’s radiation especially worthy of examination and preservation. Riparian degradation appears to have limited the range of many populations, and aquatic Hyposmocoma may serve as indicator species for assessments of riparian habitat or water quality, though habitat requirements are poorly understood.