The area of distribution of most Myriophyllum species is insufficiently known. In this paper, many new localities are recorded for 16 species from SE. Asia, Malesia, Madagascar, and Africa, and a key is added. One species from New Guinea, M. coronatum, is described as new (fig. I). Of the other species the synonymy is complete, but no descriptions are given; of each the distribution and ecology is cited, and if necessary critical remarks are added. Under the new species the second remark deals with the possible desirability of distinguishing subgenera or sections within the genus. It is concluded that, as the species show a reticulate affinity by parallelism, especially as regards reductions in both vegetative and sexual organs, the usefulness of distinguishing infrageneric taxa is debatable and not advisable.