The chemical composition of Spirulina maxima and Phormidium sp. biomasses grown on pretreated and diluted swine wastewater was determined. Analyses were carried out on lyophilized samples and compared with data from mineral media (controls). Analyses of Phormidium grown on aeration-stabilized wastewater (ASSW) were: protein (Nx 6.25) 62%, lipids 11%, carbohydrates (calculated by difference) 16%. For Spirulina in the same effluent, data were: protein 36% lipids 6% and carbohydrates 44%. No crude fiber was found in any of the samples. The fatty acid profiles of both biomasses showed important differences when compared to controls. The biomasses contained all the essential amino acids. The Spirulina biomass had a significantly higher content of pyridoxine, riboflavin and pantothenic and nicotinic acids than Phormidium when grown on ASSW, but in general the vitamin content of both biomasses was practically the same as their respective controls. The results suggest that Phormidium and Spirulina biomasses could be wed as dietary supplements in animal feed, but further studies are needed to determine the nutritional value of the product.