The reproductive biology of the Taiwanese fresh-water crab Candidiopotamon rathbunae (de Man) was investigated. The maturity, gonad development, mating behavior, breeding season, timing of release of young crabs, fecundity, and behavior of ovigerous females were studied. Mature males have sperm available for mating throughout the year. The deposition of yolk in females takes place mostly from April to June, with most of the eggs reaching maturity at the end of June. The main breeding season is from late June to October, although mating can occur all year round. In C. rathbunae, discernible courtship behavior is absent and a forced copulation strategy is practiced by males. Ovigerous females leave streams to live on the forest floor several meters away from the stream and are difficult to locate along the banks of streams. Ovigerous females return to the stream a few hours before hatching of the eggs. Most young crabs are released after mid-September, which is the end of the rainy season. This is believed to be a strategy to avoid destructive flash floods which are common from mid-May to early September. The number of eggs carried by each ovigerous female ranges 9–117, fecundity being highest for females of about 30-mm carapace width.