Morphological and molecular phylogenetic studies were made on recently collected Halymenia plants widely from Japan, the Philippines, Indonesia, Malaysia and Thailand. Although the external morphology (branching pattern, blade width, or degree of dentation) was very variable, no special differences were found in their vegetative and reproductive structures. The features are close to Halymenia durvillei. Our rbcL gene sequence analysis has shown that the branched Halymenia plants are all included in a distinct, monophyletic clade, separate from those including the foliose plants. The branched plants studied here are, therefore, concluded to belong in a single species, Halymenia durvillei, irrespective of their great external variations. As Halymenia microcarpa clearly fall within the range of external variations of H. durvillei, it was concluded to be synonymous with H. durvillei. The taxonomic interrelationship among the four varieties (var. formosa, var. ceylanica, var. denudata and var. edentata) remained unresolved, although apprently encompassed within the morphological range of H. durvillei.