A new antibiotic, aplasmomycin, which inhibits growth of Gram-positive bacteria including myobacteria in vitro, and plasmodia in vivo was obtained from a strain of Streptomyces griseus isolated from shallow sea sediment in Sagami Bay. The antibiotic forms colorless needle-like crystals and has a molecular formula of C41H60O14Na. Based on its physical and chemical properties, aplasmomycin was concluded to be a new antibiotic. The antibiotic was produced in selected media devised to relate to a marine environment.