In the course of our screening for new antitumor antibiotics from microbial metabolites, a new cytotoxic 2-o-hydroxyphenylthiazorine derivative, 4-methylaeruginoic acid (1) (Fig. 1), was isolated from the culture broth of Streptomyces sp. KCTC9303. Several members of phenylthiazoline group have been isolated from microbial metabolites. They include aeruginoic acid1}, pyochelin2), (+)-(S)-dihydroaeruginoic acid3), and several 2-phenylthiazole derivatives4^ from Pseudomonas, and thiazostatins A, B5) and ferrithiocin6) from Streptomyces. These compounds have been known as iron-chelating growth promoter2*, antibiotic3) or antioxidant5). 4-Methylaeruginoic acid is an additional member of this group with antitumor antibiotic activity. This paper describes the production, isolation, structure elucidation and cytotoxicity against several human tumor cell lines of this compound.