Nerocila fluviatilis Schiœdte and Meinert 1881 (Isopoda, Cymothoidea, Cymothoidae) is reported for the first time for a new host order of host, Mugil liza Valenciennes, 1836 (Actinopterygii, Mugiliformes, Mugilidae). Fish were collected in a coastal Atlantic lagoon in eastern Uruguay, Laguna de Rocha, and parasites were consistently restricted to the surface of caudal, anal, and paired fins. All stages of N. fluviatilis were found. Data on prevalence (6%), mean intensity of infestation (5.2), and size are provided for the first time for all stages of N. fluviatilis. An updated account of the geographic distribution and hosts of the isopod is given.