Actinoflavoside (1), a novel glycoside composed of a rare 2, 3, 6-trideoxy-3-amino-ribopyranoside (ristosamine) aminosugar and an aglycone reminiscent of the plant flavonoids, was isolated from the culture broth of a marine Streptomyces sp. The structure of actinoflavoside was established by spectroscopic analysis and by chemical degradation.