Lampetra minima, believed eradicated in 1958 and extinct, survives in upper tributaries of the historical Williamson drainage in Klamath and Lake Counties, Oregon. The species, the smallest known parasitic lamprey, was believed to be endemic to Miller Lake. Its current disjunct distribution includes Miller Creek, Jack Creek, and upper sections of the Williamson and Sycan Rivers. We compare new specimens with the type series and other Klamath Basin lampreys and redescribe L. minima. It appears most similar to Lampetra lethophaga but is smaller (72–145 mm vs 115–170 mm TL), has a larger disc length (5.0–8.6% vs 4.2–6.4% TL), larger prebranchial length (11.0–17.0% vs 8.8–13.7% TL), and larger eye (2.1–3.3% vs 1.4–2.3% TL). Klamath Basin Lampetra differ from anadromous Lampetra tridentata in a single transition in cytochrome b, and L. minima have an additional, but not unique, transition. Our data do not support the suggestion that L. minima recently evolved from a L. tridentata–like ancestor; rather we suggest a more ancient separation and a sister relationship with L. lethophaga.

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