The longitudinal distribution of fishes in the Inohzawa River watershed, southern Izu Peninsula, Japan, was documented. Water temperature and salinity in the middle and lower reaches were also measured in order to determine the influence of salt water on fish distribution. Forty-one fish species were recorded by collection with throw nets, hand nets, bare hands and angling or by visual observation, from April 2005 to August 2009. Eight additional species listed in other reports brought the total number of fish species found in the watershed to 49. Zacco platypus and Tribolodon hakonensis were abundant in the upper and middle reaches, and Mugil cephalus cephalus and Acanthogobius flavimanus in the lower reaches, suggesting that these four species were dominant in the watershed. A clear-cut salt wedge was found in the lower reaches throughout the year, the front reaching 2.6 km upstream of the river mouth. Fish species composition changed abruptly in reaches 2 km upstream of the salt wedge front, although a transition was apparent at a station 1 km upstream of the front. Therefore, the salt wedge appeared to influence fish distribution within 3.6 km upstream of the river mouth.
