A new sterculiaceous wood,Chattawaya paliformisgen. et sp. n., was collected from a Middle Eocene locality in the type area of the Clamo Formation of north central Oregon. The wood differs from all known extant woods in having very large and irregularly shaped tile cells. However, other features of the xylem indicate a close natural relationship betweenChattawayaand the extant paleotropical genus,Pterospermum. It is suggested that the xylem organization of the Eocene fossil represents a more primitive condition than that observed inPterospermum. Comparative xylotomy indicates three evolutionary trends within the complex to whichChattawayaandPterospermumbelong: development of storied structure, increased uniformity in shape and size of tile cells and loss of multiseriate rays.Chattawayais part of a growing body of paleobotanical evidence indicating that the Sterculiaceae was well differentiated by the early Tertiary, but that the lineages leading to extant genera have undergone an appreciable amount of evolutionary change since the Eocene.