The seven species of freshwater sculpins inhabiting the Japanese Archipelago (6 Cottus and 1 Trachidermus) exhibit various life history characteristics, such as catadromous, amphidromous, lacustrine and fluvial (Goto, 1990). Both the fourspine sculpin, C. kazika, and roughskin sculpin, T. fasciatus, have a catadromous life style. The former is indigenous to Japan, being found in Honshu, Shikoku and Kyushu Islands. The species migrates from the middle and lower reaches of rivers to the sea for spawning in late autumn and winter (Miyadi et al., 1976; Nakamura, 1963). However, little is known about its reproductive ecology, the only available evidence being a collection of egg clusters and attending males taken at the mouth of the Nagara River, where salinity ranged from 16 to 18 ppt (River Bureau, Ministry of Construction and Water Resources Development Public Corporation, 1992). This paper reports further aspects of the reproductive ecology of C. kazika.