Quillaja is a genus of flowering plants, the only genus in the family Quillajaceae with two or three known species. It was once thought to be in the rose family, Rosaceae, but recent research shows it belongs in its own family. The inner bark of the soap bark tree (Q. saponaria) contains saponin, which is a natural soap. Members of this genus are trees that grow to about 25 metres (82 ft).