The mechanism of trans-N-ribosylation in Corynebacterium sepedonicum was investigated. Using the DEAE-cellulose colum chromatography, this enzyme activity was divided into two fractions. One cleaved uridine to uracil and ribose phosphate, and the other decomposed inosine into hypoxanthine and ribose phosphate, in the presence of inorganic phosphate. The ribose phosphate was isolated and crystallized. Several analytical data indicated that the ribose phosphate was ribose-1-phosphate. These two enzyme fractions catalyzed the formation of nucleosides from ribose-1-phosphate and bases. Most of bacteria, which had the activity to transfer N-ribosyl group between purine and pyrimidine, could synthesize the nucleoside from base and ribose-1-phosphate.