API ZYM and API An-Ident enzymatic substrate tests were done on six oral species which are difficult to characterize with conventional biochemical tests. "Bacteroides forsythus, the "fusiform" Bacteroides species (A. C. R. Tanner, M. A. Listgarten, M. N. Strzempko, and J. L. Ebersole, manuscript in preparation), is difficult to cultivate in broth media, yet it gave 15 positive tests in these series. The tests were able to separate this new species from species of Capnocytophaga and Fusobacterium. "B. forsythus" reactions were similar but not identical to those of reference Bacteroides species. Positive reactions for alpha-glucosidase, beta-glucosidase, alpha-fucosidase, and alpha-glucuronidase suggest that "B. forsythus" may be saccharolytic. It was the only species tested which was trypsin positive. Wolinella species, Campylobacter concisus, B. gracilis, and Eikenella corrodens are asaccharolytic, and characterization relies heavily on sensitivities to inhibitory agents. These species reacted weakly in the API ZYM and API An-Ident enzymatic substrate tests, and the reactions were not useful for separating these species. The enzyme reactions differentiated Wolinella recta and C. concisus from Selenomonas sputigena, another oral motile but saccharolytic organism.