The ecologically similar species Crotonopsis elliptica and Croton monanthogynus were described by Steyermark in his Flora of Missouri as inhabitants of acid rock outcrop communities (sandstone, granite, chert) in the Ozarks but not of limestone outcrops. However, field observations and transect data confirm that C. monanthogynus can be abundant in Ozark limestone outcrop communities, codominating with Isanthus brachiatus and Heliotropium tenellum in the 5 cm to 11 cm soil depth zone, while almost never occurring on sandstone outcrops. Greenhouse experiments revealed that C. elliptica was healthy on sandstone soil, but stunted and sickly on limestone outcrop soil, consistent with its absence from calcareous substrates. In contrast, C. monanthogynus was healthy on both soils, and had greater growth on limestone than on sandstone soil. Contrary to Steyermark's characterization, C. monanthogynus grows abundantly on limestone soils, but is rare or absent on Ozark sandstone outcrops, not because of substrate intolerance, but probably because of competing species there.