Polyxylic columnar stems covered by persistent leaf bases and found in sediments assignable to the Upper Cretaceous of Bajo de Santa Rosa, Río Negro Province, Argentina, are described as two new generic entities in the Cycadales. Anatomical characters are the basis for their being assigned to the Encephalartoideae of the Zamiaceae. Brunoa santarrosensis gen. et sp. nov. is characterized by the presence of polyxyly, cone domes, mucilage cavities, and uniseriate to triseriate araucaroid, scalariform, or bordered intervascular pitting. Worsdellia bonettiae gen. et sp. nov. has polyxyly, anastomosing medullary vascular bundles, centripetal xylem, mucilage canals, and concentric extraxylary bundles. Some characters (polyxyly, medullary vascular bundles, and cone domes) were used to determine the systematic position, while other characters (mucilage reservoirs and centripetal xylem) were used to establish the relationship between polyxylic and monoxylic forms.