A new microphallid digenean, Maritrema patagonica n. sp., is described from the freshwater anomuran crab, Aegla spp. Naturally infected anomuran individuals from lakes and rivers were collected in the northern region of Argentinean Patagonia. Experimental ovigerous specimens were recovered from the intestine of chicks and from in vitro culture. Adults of the new species can be distinguished by having a small body size, papillae distributed along the edge of the forebody, unspined cirrus, and a complete vitelline ring. This microphallid is the first larval digenean species to be reported from Aegla spp.