In connection with the preparation of an account of the Nycta- ginaceae of Peru for the flora of that country now almost ready for publication, it was found that the task would be but little greater if all the plants of the family occurring in northwestern South America were treated, hence the present paper.The area covered embraces Bolivia, Ecuador, Colombia, and Venezuela, but most of the species known from the Guianas also are included.This account of the South American Nyctaginaceae makes no pretension to being a monograph, but is rather a preliminary treat- ment intended to facilitate the determination of current material received for study.The collections available are altogether inadequate to permit the study of specific variations, in which the family is notoriously prolific.I know of few groups of plants in which specific differences are so unstable and so baffling.In most of the genera, but particularly in Neea, Torrubia, and Mirabilis, no single character seems to be constant.The natural tendency, with a relatively small series of specimens, is to accord specific rank to each fairly well-marked form; yet long experience with the same family, as it is represented in the United States, indicates that more ample material will prove that 10.Neea boliyiana, sp.noy.