Two species of endangered, primarily lake-dwelling sucker are endemic to the Upper Klamath Basin in southern Oregon: shortnose sucker (Chasmistes brevirostris) and Lost River sucker (Deltistes luxatus). A third unlisted species, Klamath largescale sucker (Catostomus snyderi), also occurs in the basin. Apart from a small group of adult Lost River suckers documented in a tributary to Upper Klamath Lake in the late 1990s, it is generally believed that though the listed sucker species spawn in tributaries, the larvae out-migrate within days of swim-up, and therefore, there is no juvenile residence in the tributaries. We used X-ray imaging and vertebral counts to identify 347 juvenile suckers collected from tributaries to Upper Klamath Lake between 2006 and 2008. We positively identified 13 individuals as Lost River sucker. Our finding of juvenile endangered suckers rearing in tributaries to Upper Klamath Lake challenges the previous finding that larval and juvenile suckers only spend a small portion of their lives in rivers. This finding may have broader implications for future research and management of endangered suckers in the Klamath Basin.