Using the growing cells of Sal. typhi Sh. flexneri, Sh. sonnei, E. coli, A. aerogenes, Staph. aureus, Staph. albus and Staph. citreus, all of those were obtained from the departmental stock, the author investigated the effects of ribose, arabinose or xylose on growth as C-source and the decomposition of these pentoses during their growth. Following results were obtained. 1) Any pentoses added on the media containing the minimal essential dose of N-source and vitamins could not support the growth of Sh. sonnei and E. coli. Ribose could be served as C-source for the growth of Sal. typhi, Sh. flexneri, Staph. aureus, and Staph. citreus, while arabinose and xylose could not be so. Any pentose tested served well as C-source on the growth of E. coli when aspartic acid was added at the same time as N-source besides ammonium salt. In the case of A. aerogenes the pentoses showed the same effect even though additional aspartic acid was not present. 2) When peptone was added to media as N-source, any bacteria tested could consume these pentoses in the media. Also it was noticed that Sal. typhi Sh. sonnei, E. coli and A. aerogenes adapted to the pentoses because of their increased consumption of the sugars with the passage of culture generations. However, the adaptation was not found in the case of Staph. aureus, Staph. albus and Staph citreus. Using the resting cells of Sal. typhi, Sh. flexneri, Sh. sonnei, E. coli, A. aerogenes, Staph. aureus, Staph. albus and Staph. citreus, the auther studied oxydation of ribose, arabinose and xylose, and obtained the following results. 1) From the comparative study of pentoses oxidation abilities of the bacteria, grown on nutrient agar media, it was found E. coli and A. aerogenes had relatively great ability to ribose, however, the others showed low abilities to these pentoses. 2) In the case of Staph. aureus, Staph. albus and Staph. citreus, the oxidative abilities to the pentoses greatly depended upon the age of culture, while in the other bacteria, the ability was not so much dependable. 3) Sal. typhi, Sh. flexneri and Sh. sonnei showed slight uptake of oxygen with addition of one of the pentoses, but the uptake of oxygen markedly increased when the pentose was added with glucose at the same time. 4) When culture passed through generations on the media containing any one of the pentoses, any bacteria except Staphylococcus adapted to the corresponding sugar. 5) It seemed that the pentoses were oxidized to pyruvate by any bacteria except Staphylococcns, and pyruvate formed was in turn decomposed thoroughly, but partly it turned to and accumlated as lactate or acetate.