The influence of highly concentrated suspended solids on Ayu (Plecoglossus altivelis altivelis) survival was examined experimentally. Ayu were initially exposed to particles of eight different sizes, ranging in concentration from 1,500 to 15,940 mg/l, and the survival rate over 24 hours recorded. Subsequently, Ayu were exposed to particles of two different sizes, ranging in concentration from 560 to 20,750 mg/l, the mass and particle size distribution of particles adhered to the gills being noted. The particle size distribution of suspended solids differed from that of adherent material on the gills. A comparison of the mass of particles adhered to the gills of surviving and dead fish indicated that fatal adherence of particles occurred in a short time. The diameter of particles adhering to the gills was related to the space between two adjoining filaments. The use of particle sizes ranging from 19 to 54m m to correct suspended solid concentrations enabled the determination of 90% survival levels of Ayu under different materials and suspended solid concentrations. The results indicated that survival states of adult Ayu in highly turbid rivers can be predicted by corrected suspended solid concentrations, which can be determined for any range of particle size.