This study was undertaken to examine the effect of supplementing a suggested probiotic bacteria Lactobacillus rhamnosus JCM 1136 in feed on immune response and gut flora composition of rainbow trout Oncorhynchus mykiss. The probiotic bacteria were incorporated into a commercial feed to constitute two experimental diets containing either 10(9) or 10(11) colony forming unit of live bacteria/g of feed while a third diet without the bacterial supplement served as the control diet. The diets were offered to rainbow trout (75g average weight) in triplicate tanks for 30 days. Fish were sampled at 10, 20 and 30 days after commencement of the feeding trial to determine the proportion of the given probiont in the gut microflora composition and the nonspecific humoral and cellular immune responses on the 30th day. The relative proportion of the probiont increased with the feeding duration in the intestine, but not in the stomach. The proportion of L. rhamnosus in the stomach corresponded to the intake levels while no such relation existed in the intestine. The serum lysozyme and complement activities were significantly greater in fish fed the higher level of probiont compared with the control fish. The phagocytic activity of head kidney leucocytes also showed similar tendencies. These observations indicate the potential immuno-regulatory role of probiotic organisms in rainbow trout.