In order to determine growth of algae attached on stones in a river, a direct measurement was developed. Attached algae (10×10cm) were collected from a stone, which was randomly selected in a river. Afterwards, it was kept in a stream for one day, being surrounded by wire netting to prevent a feeding by fishes. After incubation in a river, another sample (10×10cm) was collected from the stone. The difference between the two biomass was considered as algal growth for the period. As the algal growth varied for stones, the algal biomass and its growth were fitted to a logistic curve, and the theoretical maximum biomass and growth rates were calculated. In the Rivers Yakkan, Bansho and Tsue, biomass of attached algae were 15.7g/m² (1985. July), 15.7g/m² (1985. June, August) and 4.5-13.4g/m² (1986. June, August, 1987. May, July, August), respectively, and the growth rates, 3.6g/m²/day (1985. July), 8.4, 1.7g/m²/day (1985. June, August) and 1.1-4.6g/m²/day (1986. June, August, 1987. May, July, August), respectively. Based on the theoretical value of maximum algae growth rate and the abundance of grazing marks of ayu Plecoglossus altivelis, the maximum capacity of a river for the growth of ayu was estimated: e. g. ayu would be able to grow up to 125g/m² in the Tsue River in May 1987.