The infection of the estuarine teleost fishes Mugil gaimardianus Desmarest, 1831 (Mugilidae), Arius phrygiatus Valenciennes, 1839 (Ariidae), Conodon nobilis Linnaeus, 1759 (Haemulidae), Cetengraulis edentulus Cuvier, 1829 (Engraulidae), and Anableps anableps Linnaeus, 1758 (Anablepidae) by praniza larvae of Gnathiidae (Crustacea, Isopoda) was studied in specimens fished off the Atlantic Ocean in Northeast of Pará State, near Bragança, Brazil. The highest infection prevalence value was found in Anableps anableps (42.3%) and the lowest in Conodon nobilis (9.1%). The mean intensity varied from 1 parasitein Conodon nobilis to 19.5 in Arius phrygiatus. A description of the larvae is provided. The morphology of the mouthparts is related to the blood sucking activity, and is compared with the characteristics of other gnathiidae species.