Lampetra ayresii(Günther), a species of parasitic river lamprey endemic to western North America, is redescribed on the basis of material from off the Skeena River, British Columbia, to San Francisco Bay, California. A neotype is designated. Transformed specimens differ from those of the EuropeanLampetra fluviatilis(Linnaeus) in having a larger eye, a longer prebranchial and a shorter branchial region, and possibly a lower second dorsal fin, an acute rather than an obtuse caudal fin, a higher average number of trunk myomeres, and an area of dark pigment on the caudal fin. Ammocoetes differ from those ofLampetra fluviatilisprincipally in having a higher number of myomeres, a dark caudal spot (which distinguishes them also from ammocoetes ofLampetra planeri(Bloch), the nonparasitic brook lamprey), and an absence of dark pigment on most parts of the head and on the precursor of the tongue. Specimens from Oregon are recorded for the first time. Specimens from Mill Creek, Tehama County, California, show that this species ascends the Sacramento River farther than previous records indicated. An annotated synonymy is included.Lampetra ayresiiis indicated as distinct fromAmmocoetes cibariusGirard, which is regarded as identical withEntosphenus tridentatus(Richardson).Lampetra ayresiiandLampetra fluviatilisare illustrated by a number of original drawings and photographs.