The endemic moth genus Hyposmocoma (Lepidoptera: Cosmopterigidae) may be one of the most speciose and ecologically diverse genera in Hawaii. Among this diversity is the Hyposmocoma saccophora clade with previously unrecorded aquatic larvae. I present a molecular phylogeny based on 773 base pairs (bp) of the mitochondrial gene cytochrome c oxidase subunit I and 762 bp of the nuclear gene elongation factor 1-alpha. Topologies were constructed from data using maximum-parsimony, maximum-likelihood and Bayesian search criteria. Results strongly support the monophyly of the H. saccophora clade and the monophyly of the genus Hyposmocoma. The H. saccophora clade has single-island endemic species on Oahu, Molokai and West Maui. By contrast, there are three species endemic to Kauai, two being sympatric. The H. saccophora clade appears to follow the progression rule, with more basal species on older islands, including the most basal species on 11 Myr-old Necker Island, one of the Northwestern Hawaiian Islands. Aquatic behaviour either evolved recently in the species on the main Hawaiian Islands or was secondarily lost on the arid northwestern Necker Island. The phylogeny suggests that Hyposmocoma is older than any of the current main islands, which may, in part, explain Hyposmocoma's remarkable diversity.