The nutritional quality of the blue-green alga Spirulina platerisis has been evaluated on the basis of its chemical and amino acid compositions and feeding trials with rats.The protein content of the alga was 58.5%, lysine 4.34 g 100 g-l protein, sulphurcontaining amino acids (methionine and cystine) 2.77 g 100 g-1 protein, and nucleic acids 4%. The true protein digestibility of the alga was found to be 75.5 and its biological value (BV) 68. Supplementation of the alga with methionine (0.2 %) resulted in a significant improvement in the BV and net protein utilisation (NPU), but had no effect on true digestibility (TD). TD, BV and NPU were significantly improved when the alga was supplemented with barley (50% nitrogen from both). In Chad and Mexico the alga has been used for centuries as a food. This together with its low cost of production, makes it a suitable source of high quality protein for use as a feed supplement.