Aciotis is a herbaceous genus that includes about 20 species distributed in Central America, tropical South America and the West Indies. It occurs in disturbed areas up to 1700 m elevation. Previous morphological studies of the genus have used anther size, pubescence type, leaf size, and the width of wings on the stems for species distinctions. For my revision, I have used additional characters: habit, number of pairs of paraclades of first order, inflorescence type, anther color, and presence of fibers in the hypanthium wall. Fruit characters had not been used before because it was assumed that Aciotis has only dry capsular fruits. However, field work has revealed that the genus has representatives with dry capsular fruits (e.g. A. polystachya, A. annua), species with fleshy capsular fruits (A. aequatorialis, A. acuminifolia) and species with berry-like fruits (A. rostellata, A. alata, and A. purpurascens). The capsular fruits persist on the plant, and the ovary-hypanthium walls can be caducous or persistent. The berry-like fruits fall off in their entirety when ripe. Species of Aciotis can be divided into two groups, which seem to be correlated with chromosome number and geographic distribution: 1) annuals with lax inflorescences, capsular and small fruits, low chromosome numbers and a restricted distribution; and 2) perennials with dense and conical inflorescences, berry-like and large fruits, higher chromosome numbers and wide distribution.

Key words: Aciotis, Biogeography, Melastomataceae, Morphology, Neotropics