Ctenogobius dotui Takagi is a small gobioid fish, some 30 mm in total length (Fig. 1; Takagi, K. 1957). It was collected from Ariake Sound and the vicinity of Fukuoka City, both in Kyushu. It was found to live a bottom life on sand in the coastal waters shallower than ten metres in depth. This goby resembles to Rhinogobius gymnauchen (Bleeker), syn. Ctenogobius gymnauchen(Bleeker), attaining some 60 mm in total length (Jordan. D. S. and Snyder, J. 0. 1901). Both these gobies possess a ventrally flat depressed head with eyes approaching to each other at the top of head, somewhat pointed narrow snout and large united flat vental fin with somewhat reduced thin anterior fraenum. It was observed in the natural habitat that this goby concealed itself shallowly in the bottom sand by swift dart when disturbed. The similar concealing habit was observed also in Rhinogobius gymnauchen. The sex dimorphism is remarkably shown in the form of genital papillae as well as in the dorsal fin. The three anterior spines of the first dorsal fin of the mature fish are prolonged into filaments, whereas those of the female are not. The end of the second dorsal fin of the mature male fish reaches to the insertion of caudal fin when depressed, whereas that of the female does not reach. The black nuptial coloration appeared remarkably in the mature male fish and slightly in the female. It appeared on the ventral side of head as well as the ventral, anal and caudal fins (Fig. 1). The ripe ovary contained two groups of eggs : the yellow mature one, 0.33~0.38 mm in egg-diameter, and the translucent inmature one. less than 0.15 mm in the diameter. From the similarity of the external characters, the mature ovarian eggs seemed to be spawned all at a time. The number of the mature ovarian eggs of a ripe female fish, 37 mm in total length, was enumerated as 880. The spawning-season seemed to cover all summer months in Ariake Sound. The spawning was observed to occur in a shallow tide pool on the sandy flat. The author could obtain so far only one egg-mass attached to the cleaned inner surface of a leaf of the shell of Paphia (Paratipes) undulata (Born). This dead shell, 3.5 cm in length, was found shallowly buried in the sand. The spawned eggs were shortly before hatching, hanging down from the ceiling of the shell in one layer, guarded by a parent fish staying under the shell. This spawning habit is quite similar to that of Rhinogobius gymnauchen, already reported by N. Nakamura (Nakamura, N. 1944 ; Fig. 2 A, B). The fertilized egg is club shaped demersal and adhesive, 1.88~2.06 mm in long axis and 0.41~0.47 mm in short axis, with a large perivitelline space and a bundle of a adhesive filaments at the basal end. The newly hatched was 1.9 mm in total length (Fig. 2 C). The post larvae smaller than 8.0 mm in total length were not yet obtained. In Ariake Sound the early juvenile ever 8.0 mm in total length were found already to have entered into the bottom life (Fig. 2 D, E). The gut contents of the juvenile, 8~14 mm in total length. carring bottom-life, consisted chiefly of small copepods.