An examination of the taxonomic relationships of Helianthus nuttallii T. & G. was undertaken as the second half of a detailed biosystematic investigation of the alternate-leaved perennial sunflowers morphologically similar to H. giganteus L. (Long 1960, 1961). This group includes two species of the eastern United States, H. giganteus and H. grosseserratus Martens, and three other taxa related to H. nuttallii of the western United States alnd Canada (Long 1954). It was the design of this investigation to examine the genetic comiipatibility of H. nuttallii with other species of Helianthus, especially the similar H. grosseserratus, and to gain some knowledge as to variation by measurements and comparisons of natural and garden populations with herbarium collections. Techniques used in hybridizationl experiments, analysis of natural and garden populations, and cytological observations have been described in previous reports (Long 1955a, 1960). Voucher specimens are deposited in the herbaria of IND, OWU, and USF. Helianthus nuttallii occurs from New Mexico and Arizona north into Alberta and Manitoba. It is abundant in the Rocky Mountain region where it grows up to altitudes of 9000 ft. Watson (1929) describes the plants as generally 1-2 m tall, perennial from thick, tuber-like roots, producing stems that are usually smooth and light green. The main leaves are lanceolate, average 16 cm long, 1.5-3 cm wide, light green, the lower leaves opposite, the upper alternate. Few to many heads in cymes or panicles are produced on peduncles of varying length; the phyllaries are appressed-pubescent and the disks are yellow, usually 1.5-2 cm in diameter. The eastern Helianthus grosseserratus closely resembles H. nuttallii, differing chiefly in having larger leaves that are predominately alternate and tomentose on the lower surface. Three western United States taxa allied to H. nuttallii are H. maximilianii Schrad., H. rydbergii Britton, and H. californicus DC. Helianthus maximilianii occurs in central United States and Canada, from Texas north to Manitoba and Saskatchewan, chiefly in the prairies and plains. It differs frolmi H. nuttallii by having scabrous or hispid green stemns, sessile, gray-green rough-pubescent leaves, and villous phyllaries. Helianthus rydbergii has a limited distribution in dry prairies and sandhills from Nebraska and the Dakotas into Manitoba. It is quite similar to H. nuttallii, differing only by having ovate-lanceolate leaves and pubescent stems and phyllaries. -Helianthus californicus grows in California and northern Baja California; it is closely similar to H. nuttallii except for conspicuously larger phyllaries. Population samples of H. nuttallii were collected from its central area of distribution in Colorado, Utah, and Wyoming. Table 1 gives the results of measurements of characters important in taxonomic