In our search for natural marine resources likely to be used in animal nutrition, to partially replace imported ingredients, five seaweeds were selected. The seaweeds studied were three phanerogams (Ruppia maritima, Posidonia oceanica and Cymodocea nodosa) and two green algae (Ulva rigida and Chaetomorpha linum). The chemical analysis shows that dry matter (DM), ash, crude protein (CP) and crude fibre (CF) contents vary in the five seaweeds according to the species and the season of collection. When considering the nutritional needs of Nile tilapia, Ulva rigida seems to be the best option to be included in diets for this fish. Then, we formulated four diets containing different levels of Ulva rigida: A (0 percent Ulva), B (9 percent Ulva), C (18 percent Ulva) and D (27 percent Ulva).