Light and scanning microscopies were used to evaluate the sectional boundary of 17 species included by various authors in Euphorbia sect. Tithymalopsis. Based on seed shape, surface ornamentation, and mucilage formation, five seed morphological types were recognized in North American section Tithymalopsis and related species. These results did not provide added support for recent treatment of the sort. Tithymalopsis s.s. However, a close relationship was suggested among species of the Tithymalopsis subsection Corollatae. In addition, including species from subsections Innocuae, Ipecacuanhae, and Sphaerorhizae within sect. Tithymalopsis could not be justified, based on seed morphological data.