Macrobrachium carcinus, one of the largest freshwater shrimp of the east coast of the Americas, is commercially fished in certain parts of the region. We present data concerning the reproductive biology of the species, and compare findings with those published for other species of Macrobrachium. Ovigerous females were collected monthly between March 2004 and April 2005 in the San Juan River, northern Costa Rica. Egg bearing females ranged in size from 120.1 to 190.1 mm total length. Fecundity increased with female size and, up to 242,437 eggs, can be considered as high compared to other species of Macrobrachium. Average egg volume increased gradually during embryogenesis from 0.065 to 0.088 mm³, representing an overall increase of 35.4%. Eggs can be considered as small, typical for Macrobrachium species depending on brackish water to complete larval development. Reproductive output of M. carcinus varied between 4.0% and 21.0%, and was independent of female size; these values are within the range reported for other species of Macrobrachium and other marine caridean shrimp.