Nichol's turk's head cactus Echinocactus horizonthalonius var. nicholii (Cactaceae) occurs in a few isolated populations in the Sonoran Desert of south-central Arizona (Pima and Pinal counties) and in one isolated population in Mexico. Populations of this variety are disjunct from the more widespread typical variety (E. h. var. horizonthalonius) that occurs in the Chihuahuan Desert of New Mexico, Texas, and Mexico. Variety nicholii occurs almost exclusively on Horquilla limestone substrate, and was federally listed as endangered in 1979. Since 1995, we have monitored study plots of this cactus in the Waterman Mountains, Pima County, Arizona, 32°20′N, 111°28′W, elevation 975 m, measuring size of plants, condition, and presence-absence of reproduction in ca. 200 individuals. Here we report data for 1995–2008. Rates of growth were slow, averaging 0.35 cm (SD ± 0.53) in height and 0.26 cm (SD ± 0.28) in diameter/year. Plants usually began flowering when they reached about 4 cm in height and 8 cm in diameter. Of 109 deaths recorded since 1995, 47% followed a visible decline in condition; the remainder of deaths were unanticipated. The study population declined from 129 individuals in 1995 to 89 in 2008.