The grain of eelgrass (Zostera marina) consumed by Seri Indians of Sonora Mexico is the only known case of a sea grain used as human food. Z. marina was combined with the seed of Cardon (Pachycereus pringlei), a columnar cactus of the Sonoran Desert. Protein quality was determined by the NPR and RNU methods. The NPR results showed significant diffeiences (p<0.05) between raw and toasted Z. marina that could be related to the lysine lost in the toasting process. P. Pringlei, as well as the blend, showed lower values (p<0.05) than the grain of Z. marina, possibly due to high fiber content or the presence of antinutritional factors. Additionally, the iron and riboflavin content seem to be considerable.