The morhology and life history of Stylonema cornu-cervi Reinsch from Japan were investigated. The species had multiseriate erect thalli from a basal cell. The thalli usually branched dichotomously, occasionally trichotomously near the base, and non-branched thalli were sometimes observed. A dichotomous branch on the upper portion near the base occurred only one time on each erect branch. Cells contained a stellate chloroplast, which was composed of a central rounded part with an obscure pyrenoid and 5-8 cup-like lobes connected to the central part by a small thin stipe. The biseriate part was observed on the six-celled stage in culture, and the grown thalli were multiseriate except for base and apices. Monospores forming from the immediate transformation of vegetative cells were observed. Thalli grew at 15-25$^{circ}C$ and died at 10 and 30$^{circ}C$. The fastest growth and maturation were observed under 25$^{circ}C$ and 14L:10D. Although S. alsidii (Zanardini) Drew usually had uniseriate thalli, irregularly branched multiseriate thalli had been reported in cultures. It is possible that in the previous report the thalli were confused with S. cornu-cervi. In this report, S. cornu-cervi were distinguished from S. alsidii in that the branches were few, the multiseriate portions were observed on the early stage (six-celled stage), and the grown thalli were multiseriate except at the base and apices.