Digestibility of the edible brown alga kombu was studied by feeding rats for 28 days. Food intake of both rats fed with basal and kombu diets was statistically similar. Body weights of kombu groups were significantly lighter, but their fecal weights were heavier, when compared with their counterparts with diets excluding kombu. There were no major differences in the weights of liver, pancreas, and kidney, but cecum, small intestine, and large intestine of rats fed with kombu were significantly heavier than those without kombu. With the kombu diet, in the early feeding period protein digestibility was signi-ficantly lower than in the control, but the digestibility of kombu groups recovered to the level of the control in the late feeding period. Fat digestibility in the kombu diet was significantly higher than that in the basal diet. Digestibility of insoluble and total dietary fibers in the kombu diet tended to increase in the feeding period. Alginate digestibility significantly increased, and the molecular weight of alginate was down by 60%. Since the molar ratio of mannuronic and guluronic acids in alginate decreased significantly in the feeding period, mannuronic acid may be more decomposable in comparison with guluronic acid in rats.