Abstract The genus Diodella of Small is not synonymous with Hedyotis § Diodella as is indicated in Index Nominum Genericorum. The two taxa have different nomenclatural types and belong to different tribes of the Rubiaceae. The first mentioned has a single seed in each locule and the second has many seeds. The stipules and seeds also differ markedly. Consequently the spermacocoid genus Diodella (typified by the type of Diodia rigida) is strongly distinguished from Hedyotis § Diodella [typified by the type of Hedyotis boscii DC. = Oldenlandia boscii (DC.) Chapm.] by many significant morphological features and should not be confused with it. The type of Small's genus Diodella is the type specimen of D. rigida, a perennial species not known to occur in the United States, although mistakenly included by Long and Lakela as well as by Small. The closely related but annual Diodella teres is common and widely distributed throughout much of the eastern United States, Mexico, Central America, and the Greater Antille...