Movements, ranging behavior, and social organization of the ricefield rat (Rattus argentiventer) were studied by radiotracking in a rice field in West Java, Indonesia. Home ranges were estimated by the minimum convex polygon method and were found to be significantly larger in the nonbreeding season than in the breeding season. During the breeding season, males had larger home ranges than did females (3.20–3.24 ha, as compared to 2.51–2.34 ha), but no sexual differences were found in the nonbreeding season. Overlap between home ranges occurred during the breeding season, especially for males. During the nonbreeding season, rats appeared to be nomadic, and nests seemed to be distributed at random within ranges. Males and females were never found to share nests during the nonbreeding season. Habitat use was investigated by recording percentage occupancy of the following habitat elements: rice paddies, banks, and vegetation. Totals of 63% and 60% of radiofixes were made in paddies during the rainy and dry seasons, respectively. Paddies were probably used for feeding, nest construction, and protection from predation.