During the course of our screening program for antibiotics specifically active against Pseudomonas, we discovered four such new compounds which we named napsamycins A~D,1} from the culture filtrate of a Streptomyces sp. HIL Y-82,11372 (DSM accession No. 5940). Chemical and spectroscopic analysis revealed that they belonged to the class of peptidylnucleoside antibiotics represented by the mureidomycins2'3) and pacidamycins.4'5) Herein we report the production, isolation, structural elucidation and biological activity of the napsamycins.