Pot experiments were conducted to investigate the dynamics of N derived from sewage sludge (N : P : K = 6.9 : 3.1 : 0.4 %) applied in a low-fertile paddy soil by using ¹⁵N - dilution method at the University Farm, Faculty of Agricultural Ehime Univ., Matsuyama in Japan. The properties of soil used was follows : Brow forest soil, Inceptisols Umbrepts, T-C = 1.32%, T-N = 0.09%, pH = 6.2, EC = 0.065 dS m^-1. Wagner pots (0.02 m²) containing 3.37 kg of dry soil were fertilized with chemical fertilizer (N : P : K = 7.0 : 3.9 : 7.5 g m^-2), sewage sludge (N : P : K = 15.2 : 5.2 : 1.0 g m^-2) or no fertilizer (control). Potassium in the sewage sludge plot was supplemented with KCl (K = 7.5 g m^-2). For tracing the fertilizer N, the chemical fertilizer plot was amended with ¹⁵N labeled fertilizer (¹⁵NH₄Cl, 1.00 atom%) solution at transplanting time (June 10), tillering stage (June 29) and young panicle formation stage (July 27) at the rate of 4 g, 2 g and 2 g N m^-2, respectively. In the sewage sludge plot the sludge was applied as a basal fertilizer, and ¹⁵N tracer (0.1 g m^{-2 15}NH₄Cl, 99.8 atom %) was injected to the soil. After amending the chemical fertilizer and sludge to the soil, pots were flooded with tap water, and three seedlings per pot of rice (Oryza sativa L. cv. Koshihikari ) were transplanted. From transplanting to harvest, the pots were placed in the greenhouse. The fate of N from the sludge was estimated by ¹⁵N - dilution method. During the cultivation the plant height in the sludge plot was significantly (p = 0.05) lower than that in the chemical fertilizer plot. The number of tillers in the sludge plot was also significantly lower than that in the chemical fertilizer plot at 17 d after transplanting, but there was no significant (p = 0.05) difference in the number of tillers at maximum tillering and ripening stage (p = 0.05). The leaf greenness in the sludge plot changed similar to that of the chemical fertilizer plot. During the cultivation, the significantly (p = 0.05) lowest plant height, number of tillers and leaf greenness were observed in the control. At harvest the rice dry weight was significantly (p = 0.05) higher in the chemical fertilizer plot than that in the sludge plot. In the chemical fertilizer plot the rice plant absorbed 132 mg pot^-1 of N derived from the fertilizer, it accounted for approximately two third of the amount of the chemical fertilizer N mineralized (200 mg pot^-1). In contrast the rice in the sludge plot absorbed 117 mg pot^-1 of N derived from the sewage sludge, it corresponded to approximately one third of the amount of the sewage sludge N mineralized (334 mg pot^-1). In the sludge treatment 101 mg pot^-1 of N derived from the sludge remained in the soil and it was 10 times higher than that from chemical fertilizer (9.20 mg pot^-1). The amount of unaccountable N from the fertilizers such as denitrification and NH₄ volatilization between chemical fertilizer and sewage sludge treatments were not significantly different. Soil microbial activity in the sludge plot, measured by an enzymatic assay (esterase activity to fluorescent diacetate), tended to be higher than that in the chemical fertilizer and control plots suggesting a higher nutrient cycling in the sludge treatment. Higher amount of mineralized N from sewage sludge was considered to be absorbed and metabolized by soil microbial biomass and resulted in the high amount of N remained, leading to increase in soil fertility. Nitrogen use efficiency in the chemical fertilizer and sewage sludge plots were estimated to be 65.9 % and 38.7%, respectively. As a result, the relative efficiency in the sludge plot was 58.7 % of the chemical fertilizer. It suggests that the sewage sludge N should be applied 1.7 times the amount of chemical fertilizer N to achieve the same efficiency. Soil pH in the chemical fertilizer plot (7.02) was significantly (p = 0.05) lower than that in the sludge plot (7.51), the EC of soil in the chemical fertilizer plot (0.780 dS m^-1) was significantly (p = 0.05) higher than that in the sludge plot (0.275 dS m^-1). The higher EC in the chemical fertilizer plot may have resulted from the higher amount of chemical fertilizer existing as soluble ions. Concentrations of heavy metals such as Cd and Zn were negligible low in the sludge. It is concluded that sewage sludge is evaluated as a useful organic fertilizer from the viewpoint of fertilization, healthy growth of the crop and nutrient recycling in the environment.