A diverse assemblage of silicified woods occurs in the type area of the Clarno Formation in north central Oregon, USA. These woods occur as predepositionally abraded fragments along with abundant fruits and seeds in tuffaceous sediments at the Nut Beds locality in the Clarno Unit of John Day Fossil Beds National Monument. A comprehensive investigation of these woods, involving the thin-sectioning and analysis of more than 500 specimens, has resulted in the recognition of about 70 distinct taxa--making it the richest Eocene wood assemblage so far known. Although the diversity of woods at the Nut Beds apparently is not as high as that of fruits and seeds (ca 175 species), the woods document some new families for the flora and provide improved insight into the composition of this thermophilic assemblage. At least fifty of Nut Beds wood types are referable to extant family. Among these, some have anatomy diagnostic of particular modern genera (e.g., Alangium, Meliosma, Prunus); others represent extinct genera (e.g., Clarnoxylon--Juglandaceae, Triplochitioxylon--Malvaceae s.l.); but most are stereotypic genera, i.e., cannot be assigned to an extant genus because their combination of features occurs in more than one extant genus (e.g., Magnoliaceoxylon, Maloidoxylon). We recognize the following families based on woods from the Nut Beds: Aceraceae, Anacardiaceae, Annonaceae, Araliaceae, Betulaceae, Cercidiphyllaceae, Cornaceae, Eupteleaceae, Fagaceae, Flacourtiaceae, Hamamelidaceae, Juglandaceae, Lauraceae, Leguminosae, Magnoliaceae, Malvaceae, Moraceae, Platanaceae, Rosaceae, Sabiaceae, Sapindaceae, Ulmaceae, Vitaceae, Palmae, Ginkgoaceae, Pinaceae, Taxodiaceae.

Key words: Clarno Formation, Eocene, fossil wood, paleobotany, Tertiary, wood anatomy