This is the fifth in a recent series of papers on the poorly known western North American pebblesnail genus Fluminicola (Caenogastropoda, Lithoglyphidae). Herein we clarify the taxonomic status of the currently undescribed pebblesnail fauna in the upper Klamath River drainage (UKL) based on morphologic evidence, and mitochondrial DNA sequence data from 58 UKL collection localities. We describe one new species (F.klamathensis) from eight UKL localities which is differentiated by mtDNA sequences and unique penial morphology, and document range extensions to the UKL for three species from closely proximal drainages (F.fresti, F.modoci, F.multifarius). Fluminicolafresti was found at a single locality along the western edge of upper Klamath Lake. Fluminicolamodoci and F.multifarius are widely distributed in the UKL; both species exhibit marked morphologic variation yet are relatively little differentiated genetically in this basin.
Keywords: Caenogastropoda; Truncatelloidea; Pacific Northwest; freshwater; mitochondrial DNA; morphology; systematics.