The teeth, teeth-bearing bones and the teeth-band-folds coverirg the vomer and palatines on the roof of mouth were investigated for using as a taxonomical character. Their arrangement, forms and connective conditions were compared among 8 species belonging to 3 genera in Japan (Table 1.) and the characters of them showed to admit the determination of each species. The results are summarized as follows.<BR>(1) Vomer and hyoid bone are more characteristic for determination of genus and species of Salmonoid fishes than the other bones of mouth cavity (Figs. 2, 3). The number of teeth on the vomer, palatines and tongue was less adequate as taxonomical character, because the teeth often disappear in the spawning season and the number of teeth reduces with age and shows wide individual variations.<BR>(2) The author could not find individual variations and changes by age and sex in the arrangement, forms and the connective conditions of the folds on the vomer and palatines and in the form of labial fold (Plate 1, 2, 3). Therefore these could be adopted as the taxonomical characters of genera and species of Salmonoid fishes.<BR>(3) Three genera of Salmonidae can be identified easily by the arrangement of the teeth-band-folds, i. e. Oncorhynchus shows +-shape (Fig. 1, A-D), Salmo T-shape (Fig. 1, E, F) and Salvelinus M-shape (Fig. 1, G, H).<BR>(4) Each species, in Oncorhynchus (Fig. 1, A-D) and Salvelinus (Fig. 1, G, H), can be identified by the forms and connective conditions of the folds on vomer and palatines and the form of labial fold. In Salmo (Fig. 1, E, F), besides the abovementioned character, each species can be identified by the forms of teeth on maxillaries and palatines.