Two years after its first appearance in the marsh of the Doñana National Park, the survey undertaken by the Doñana Monitoring Team (ICTS-RBD-CSIC) confirmed that the blue crab, Callinectes sapidus Rathbun (1896), is present in large numbers in the Brenes and Figuerola channels. In the surveys of May in these two locations, 14 specimens (9 females and 5 males) between 5 and 148 grams of weight were captured, confirming the establishment of the species in the area. Originally from the western Atlantic coast, from Argentina to Nova Scotia, It was into European waters at the beginning of the 20th century, and on the Spanish Mediterranean coasts at the beginning of this decade. This species lives in coastal areas and shallow inland waters, preferably with abundant vegetation, feeding mainly on invertebrates, carrion and small fish. To complete its biological cycle, however, it requires saltier and deeper waters, so the females spawn offshore, releasing over two million eggs that will return to the estuaries and marshes to feed and grow once reached the juvenile phase. Although the impact in the marshes of the Natural Area has not been established yet, It is known to affect negatively other decapod populations such as the green crab Carcinus maenas, which is common in the estuary.