The trial was made to study the protein digestion in juvenile turbot during passage of digesta from stomach to rectum. Moreover, diet supplementation with the potential probiotic bacteria, Vibrio proteolyticus, was evaluated with regard to protein digestion. For a 3-week period, fish (25-30 g) were fed by oral intubation and received either a liquid mixture consisting of 40% nonpurified control diet and 60% water or this mixture supplemented with 10(10) viable V. proteolyticus ml(-1). Daily dry matter intake was 1.5% body weight. As digesta progressed from the stomach to the foregut, hindgut and rectum, the increase in apparent nitrogen digestibility was accompanied by higher ammonia contents, suggesting substantial involvement of the microbiota in protein degradation in the distal segments of the gastrointestinal tract. Water-soluble nitrogen contents were significantly higher in the foregut, presumably corresponding with considerable protein digestion by secreted endogenous enzymes in this digestive segment. Over 65% of the soluble protein in all four parts of the tract had a MW < 10,000. The amount of soluble protein and peptides with MW < 1000 decreased significantly during transit. This was also found for the proportion of the 10,000-20,000 MW proteins, whereas the highest MW category (> 200,000) increased. Ingestion of V. proteolyticus tended to stimulate apparent nitrogen digestibility (P < 0.1). This effect corresponded with increased protein degradation in the proximal intestine as was shown by the significantly elevated fraction of soluble proteins with MW < 1000. (C) 2000 Elsevier Science B.V. All rights reserved.