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A. orally dimorphic hexaploid (n = 33), Echinocereus yavapaiensis M. A. Baker, is newly described and possesses the highest number of chromosomes known within the genus. Populations of E. yavapaiensis occur in central Arizona between 1025 and 1860 m elevation and are associated with volcanic bedrock. Fifty three percent of the individuals studied produce only pollen-sterile flowers, which fail to undergo meiosis within the micro-spore mother cells, while 47% develop only pollen-fertile flowers. Chromosome determinations are presented for 41 populations of Echinocereus, including E. arizonicus (n = 11), E. canyonensis (n = 22), E. coccineus (n = 22), E. matudae (n = 11), E. pacificus (n = 22), E. mombergerianus (n = 22), E. santaritensis (n = 22), and E. yavapaiensis. A phenetic analysis compares morphological variability among populations of E. yavapaiensis to those of its closest allies.