This study analyzes the spatiotemporal variation of the ichthyofauna of the surf zone of Praia do Cassino, municipality of Rio Grande (state of Rio Grande do Sul, Brazil) from October 2005 to September 2006. Fishes were collected monthly and during daytime using a large beach seine net (30 m x 1.8 m, with 12 mm uniform mesh manufactured with monofilament nylon wire). Five hauls were conducted on each of three sampling sites: one near the jetties of Patos Lagoon and two located 5 km and 10 km south from the jetties, respectively. We caught 10,066 fishes, weighting 128,486 g, distributed in 37 species belonging to 18 families that are similar in species composition to the dominant fish fauna - Trachinotus marginatus (Cuvier, 1832), Atherinella brasiliensis (Quoy & Gaimard, 1825), Brevoortia pectinata (Jenyns, 1842), Mugil platanus (Gunther, 1880), Menticirrhus americanus (Linnaeus, 1758), Menticirrhus littoralis (Holbrook, 1847), and Oncopterus darwinii (Steindachner, 1874) - usually collected in the beaches of Rio Grande do Sul with a smaller net. The dominant size classes were between 50 and 100 mm TL and represent 43% of total catch. Based on CCA analysis we suggested that the ichthyofauna is primarily structured by seasonal variation in the physicochemical and environmental parameters and secondarily by the spatial difference between areas nearer (< approximately 10 km) or more distant (> approximately 10 km) from the Patos Lagoon's jetties. The composition and dominance of the fish community of the surf zone of Cassino Beach changed little in the latest 20 years, indicating a possible stability of the system.