Protein digestibility and protein quality of raw rainbow trout, broiled rainbow trout and smoked rainbow trout were studied by in vitro assay, Amino acid score (AAS) and protein digestibility corrected amino acid score (PDCAAS). Protein digestibilities of samples were determined using an in vitro, three-enzyme method in a pH-stat and three- and four-enzyme pH-drop methods. Amino acid score was based on the amount of the single most limiting amino acid, and its calculation included the use of the requirement pattern suggested by FAG/WHO/UNU for pre-school children. Protein digestibilities of raw, broiled and smoked rainbow trout were found to be 87.1, 84.0 and 83.4% using the three-enzyme pH-drop method, 84.7, 81.4 and 80.8% using the four-enzyme pH-drop method, and 95.5, 93.9 and 91.2% using the three-enzyme pH-stat method, respectively. When the amino acid score was corrected for in vitro (three-enzyme pH-stat method) protein digestibility, the resulting values of 99.8, 97.1 and 93.9% were obtained. Amino acid score corrected for protein digestibility seems to predict, accurately, the nutritional quality of fish protein when in vitro values are used.