A new antibacterial antibiotic tetrazomine was found from the fermentation broth of an actinomycete strain which was isolated from beach sand collected at Chichijima, Ogasawara Islands, Tokyo, Japan. The strain Y-09194L, was identified as Saccharothrix mutabilis subsp. chichijimaensis subsp. nov. The antibiotic exhibited broad antimicrobial activity against Gram-positive and Gram-negative bacteria in vitro. It also exhibited strong cytotoxic activity against P388 leukemia cells and showed antitumor activity against P388 leukemia. The apparent molecular formula of tetrazomine was determined as C24H34N4O5. It has a rare structure which consists of six rings including piperidine, piperadine, oxazole, and pyrrolidine.