Abstract. Caged fish farming at Riam Kanan River, South Kalimantan has been done by the local farmers since 1986. The huge profit from this business triggers exploitation of the rivers without environmental consideration. As a consequence, it increased the fish mortality rate and the decrease of fish farmers’ income. In order to control the number of fish cages, a study in regard of its condition and/or carrying capacity is needed. The objective of this study was to estimate the amount of phosphorus (P) waste produced by fish farming activity. The research was conducted using six cages with the size of 2.5 x 1.75 x 1 m3 : three cages with capacity of 220 fish per cage and three cages with capacity of 500 fish per cage. Cultivated fish is common carp (Cyprinus carpio L.) with the size of t 5-7 cm long. Two times feeding per day with 5% body weight was applied. The cultivation was done for 16 weeks. The daily distribution of P of the caged fish farming was calculated based on the total amount of leftover food and the feces in 12 hours. The weight of leftover food and feces, collected from the net surrounding the cages, were sampled every two weeks. The result indicated that total P increased with the length of fish cultivation. The total amount of P in every cage with the capacity of 220 and 500 fish were approximately 0.035 g day-1 and 0.207 g day-1 , respectively. Hence, the optimum number of cages on Riam Kanan River based on the existing P was as follow, cages with 220 and 500 fish were 60-64 and 10-12 units per 100 m, respectively. Key Words: water pollution, sustainable aquaculture, river carrying capacity, cage fish farming,