Conversion of oil-rich salad oil manufacturing wastewater (SOMW) into protein source for animal feed through biomass production of yeast isolate was investigated in this study. Five species of yeasts, including Rhodotorula rubra, Candida tropicalis, C. utilis, C. boidinii, Trichosporon cutaneum, were isolated from SOMW following enrichment culture. Of them, C utilis was chosen as the sole biomass producer in the study due to its greatest oil uptake rate, 0.96 kg oil kg(-1) biomass d(-1), and highest specific growth rate, 0.25h(-1). The cells of C utilis contained 26% protein, 9% crude lipid, 55% carbohydrate and balanced amino acid compositions. The initial N:C ratio in SOMW drastically influenced oil reduction efficiency, biomass production and protein content of C utilis, and therefore a range between 1:6 and 1:8 was recommended in consideration of these three factors simultaneously. (c) 2004 Elsevier Ltd. All rights reserved.