The reproductive biology of the crane river prawn Macrobrachium formosense Bate (Palaemonidae) was studied based on a large number of specimens which were sampled during a 12 month period in the Yabusa River in southern Kyushu, Japan. The carapace length at sexual maturity for females was estimated to be 10.20 mm. Ovigerous females were found from May to September. The spawning and hatching periods were estimated to last from May to August and from June to September respectively, based on the occurrence of ovigerous females with first stage and last stage eggs. M. formosense in the river was considered to have multiple spawns during a single annual breeding season because ovigerous females which spawned early in the breeding season were found to have a high value of the gonadosomatic index. Egg size was 0.522×0.429 mm at spawning and 0.651×0.523 mm just before hatching. The estimated breeding season, egg size and maximum body size of ovigerous females of M. formosense in the Yabusa River were considerably different from those in the Ryukyu Islands.