Acentrogobius masago (Tomiyama) is a small gobioid fish growing about 25 mm in total length ; it is distributed in Middle and Southern Japan, living a bottom life on sandy-mud bottom in estuaries (Fig. 1). The author collected over 700 individuals of this gobioid fish, including growth stages, from Kyushu and Inland Sea, and examined the external and internal body-features, e. g. number of fin-rays, scale-phases, tongue, alimentary duct, genital organs, number of vertebrae, etc. (Fig. 2). The sex-dimorphism remarkably developed in the form of genital papillae. The mature female fish is generally larger than the male (Fig. 3). The black nuptial coloration appeared only in mature male fish on the second dorsal and anal fins. The ripe ovary contained two groups of eggs : ones, 0.48~0.57 mm in diameter, and the immature ones, less than 0.11 mm. The number of the mature ovarian egg of one individual was enumerated as 264~961 in 14 specimes (Table 1) ; and the mature eggs seemed to be spawn all at a time. The spawning-season seemed to extend from May to September in the vicinity of Fukuoka City, Kyushu. The spawning-behaviour in natural habitat is not observed. The fertilized egg by artificial insemination is spindle in shape, 1.2 mm in long axis, and 0.4 mm in short axis, with a bundle of adhesive filaments at the basal end. In early developmental stages, 3 8 oil-globules appeared in the translucent yellow yolk (Fig. 4). The incubation-period was about 4 days at the water-temperature 21-25℃. The newly hatched larva is 2.0 mm in total length (Fig. 5). The prelarvae, 6~8 mm in total length, appeared from June to September in the vicinity of Fukuoka City, living a swimming-life in the littoral zone. The juveniles, 8~10 mm in total length, were found to live a bottom life in estuaries (Fig. 5). The gut-contents of the post-larvae, carring a swimming-life, consisted chiefly of planktonic copepods, whereas those of the young and adult fishes, living a bottom-life, mainly of organic detritus on the bottom. The examination of the size-frequency of the collected specimens shows that this gobioid fish attains 16~25 mm in total length, and the most individuals become mature in a year. The minimum adult seems to be 17 mm in total length in both sexes, and the life-span over 2 years.