Among the extensive collections of algae made by Dr P. Wagenaar Hummelinck (Utrecht) in the Antilles and adjacent regions during the years 1930, 1936, 1937, 1948—1949, 1955, a number of chiefly brackish, but also freshwater, Cyanophyceae were incorporated. This collection was kindly committed for study to the author. She is indebted to Dr F. Drouet, who identified part of it. Most of the localities have been amply described by the collector (1940, 1953), who also included pictures of several localities in the same papers. The map illustrating this paper was drawn by the collector. He also was so kind as to complete the descriptions of the localities in the present paper.