Detailed morphological studies were carried out on the vegetative thallus and female, and tetrasporangiate systems of the red alga, Hyalosiphonia caespitosa Okamura (family Dumontiaceae). As a result, the following data were obtained : 1) each segment cell produces four vegetative laterals, of which the distal ends form a tight cortex, 2) basal cells of the cortex bear a carpogonial branch and an auxiliary cell branch separately, 3) both carpogonial branch and auxiliary cell branch are strongly curved and 4) after fertilization, the carpogonium fuses with a cell of a carpogonial branch and they form a fused area from which develops a connecting filament which ultimately fuses with the auxiliary cell. These features support well the systematic position of
the genus Hyalosiphonia as a taxon of the Dumontiaceae.