Spawning of Ayu, Plecoglossus altivelis, downstream migration of larvae and upstream migration of juveniles were investigated in the Murasaki River, Kitakyushu. It became clear that the number of downstream larvae at the fishway of the weir for industrial use were very few; their ratios to the number of downstream larvae at the point of just under the spawning ground were ranged from 1,1% to 2.5% (mean 1.8%, n=4). Because the density of larvae in the bottom water layer at the weir was distinctly higher than that in the surface, the fishway equipped with slide gate dischargeable the bottom water is necessary.