The chemical composition of red seaweed Gracilaria cervicornis and brown seaweed Sargassum vulgare from Brazil was investigated. In this study, the relationship between the nutritive components of each species and the environment was established. Protein content varied from 23.05+/-3.04% to 15.97+/-3.04%. The highest value was found in G. cervicornis. The protein levels were positively correlated with nitrogen content and negatively with water temperature and salinity. Carbohydrate contents of both species varied significantly (p<0.01) and the values observed were superior to others chemical constituents. Contrary to carbohydrates, the lipid concentrations were the lowest recorded chemical component and varied slightly between the two species. Ash content was greater in S. vulgare (14.20+/-3.86) than in G. cervicornis (7.74+/-1.15). In general the variation in chemical composition was related to environment.