Over 100 species have been described in the Hookeriaceous genus Cyclodictyon. Of these, 13 have been described as being synoicous and included in the present study which represents the first part of a monograph for the whole genus. Greatest diversity occurs in tropical Latin America with a second center of speciation in tropical Africa. A single species with several varieties extends across Malesia from Tahiti westward to Ceylon and northward into the Philippines and Taiwan. The Malesian Cyclodictyon blumeanum is synoicous as well as several species each in continental South America and Africa. Synoicous species are so far unknown in Central America or the Antilles. Recognition of taxa depends mainly on combinations of vegetative characters including size, length-width ratios of leaves, costal morphology, leaf border morphology and areolation. As a result of examination of types and other material of synoicous species, 10 are recognized, one is placed in synonymy and 4 have been reduced to varietal status.

Key words: Cyclodictyon, Hookeriaceae, moss taxonomy, tropical moss