Nybomycin (1) and deoxynybomycin (2) comprise a thus-far unique structural class of antibiotics that contain a linearly fused pyridoquinolone ring system and an angularly fused oxazoline unit. Recently, we isolated a third member of this unusual class, hydroxynybomycin (3), which occurs as a minor metabolite of the nybomycin-producing organism, Streptomyces sp. D-57. Its isolation, structure elucidation and bioactivity are reported herein.