AbstractSaccharomyces fragilis (NRRL y 1109) was grown on a medium composed of Cheddar cheese whey ultrafiltration permeate, supplemented with inorganic nitrogen. The biomass had a crude protein content of 49.5% and an ash content of 7.6%. The “true protein” content, based on a N fractionation procedure was 37.2% and the total nucleic acid content was 5.7%. Qualitatively, the amino acid composition of S. fragilis was similar to that reported for other whey‐yeasts. The quantity of individual essential amino acids was greater in the permeate grown S. fragilis. The sulphur containing amino acids, methionine and cyst(e)ine were present at a combined level of 3.2 g/16g N, compared with 2.2g/16 g N in a commercial whey‐yeast product. The chemical score (CS) and essential amino acid index (EAAI) were 58.2 and 76.0 respectively. Rats fed on a diet containing S. fragilis biomass as the sole source of dietary protein showed a net weight increase. NPUcarcass for S. fragilis protein was 67.0±3.9 compared with a value of 75.3±3.0 for a casein control. Supplementation of S. fragilis biomass with DL‐methionine(2 g/16 g N) raised the NPUcarcass value to 84.4 ± 2.8.