The antibiotic nucleocidin1) represents highly cytotoxic 5'-O-sulfamoyl nucleosides whose sulfamoyl group is believed to simulate a phosphate group. Because of its tin-ionized nature, the molecule can cross cellular membranes',''. Recently, TAKAHASHI and BEPPU reported a nucleoside antibiotic AT-265, and the structure was proposed as 5'-O-sulfamoyl-2-chloroadenosine on the basis of spectral evidence'. However, no direct proof was given regarding the configuration of this compound. More recently, we have isolated two nucleoside antibiotics from a fermentation broth of a Streptomyces. One of them was found to be identical with antibiotic AT-265, however, the other one was the structurally remarkable alanyl derivative of AT-265 and designated as ascamycin. We report here isolation, structure, and biological properties of these antibiotics.