Clarnoxylon blanchardii gen. et sp. nov. is a new taxon for fossil wood with a suite of features diagnostic of the Juglandaceae. It occurs at two Middle Eocene (c. 43-44 million years b.p.) localities in the Clarno Fonnation of central Oregon, USA. Clarnoxylon resembles the Platycaryeae and the Hicorieae in having exclusively simple perforation plates and solid pith. However, the common occurrence of crystalliferous idioblasts in the rays, but not in the axial parenchyma, and the cooccurrence at Clarno of platycaryoid fmits and pollen unaccompanied by hicorioid fmits indicate that Clarnoxylon has affinities with the Platycaryeae. Differences between Clarnoxylon and Platycarya support previous suggestions that short vessel elements, helical thickenings, and vascular tracheids are derived characters of Platycarya. These differences are also in accord with the ecological adaptation of the extant genus Platycarya to a temperate climate contrasting with the tropical Middle Ebcene setting of Clarnoxylon.