The distinctive characteristics of Hemirhamphus kurumeus (JORDAN et STARKS) described by the original authours from the sommon Japanese halfbeak, Hemirhamphus sajori TEMMINCK et SCHLEGEL, are not always quite clear when many specimens of both species are examined. Chiefly based upon this fact the two named species are considered to be inseparable by some writers as Shigeho TANAKA^<4)> (1931) and Ichiro TOMIYAMA^<7)>(1934). In the present paper the writer has statistically treated some morphological characteristice of the two species which may serve to specifie distinction, and further made some comparison of minor characteristics and ecological aspects as well as the developmental stages. Table I and fige. 1-6 show the characteristics statistically treated and the results obtained from the comparison of them. In the number of seales in longitudinal series, number of vertebrae, and the formula and total number of gill-rakers (of the first gill-arch), the ranges of variation and modal classes of the two species are shown to be independent reciprocally. Among them, the number of vertebrae may be considered as a decisive feature in specific distinction.Among the characteristics not statistically treated, the size of adult fish, length of pectoral fin, melanophore arrangement on the scales of the dorsolateral side of body, and the coloration of the underside of tip of lower jaw (in fresh state) seem to provide good distinctive clues. As to the ecological aspects, H.kurumeus lives chiefly in the brackish or fresh water, whereas H.sajori lives usually in the sea. A landlocked case of the former species has recently been found in southern Korea in an absolutely fresh water swamp in which it propagates well. The early larva carrying its yolk of H.kurumeus is considerably smaller than that of H.sajori (Fig.7 A, B). Subsequent developmental stages of both species bear striking resemblances in form and coloration, but can be easily di stinguished by the enumeration of the myotomes (or vertebrae). From the results of these studies, the present writer has reached to the conclusion that these two must be considered specically independent with each other and can not be regarded as mere variants belonging to one and the same category of systematic unit.