In the course of screening microorganisms for the production of anticoccidal and anthelmintic agents, a Streptomyces sp. designated LL-F35101 was isolated. This culture subsequently appeared to be identical with Streptomyces albus NRRL 3384, the producer of polyether antibiotic A204 A. Culture LL-F35101 was found to produce a new anthelmintic antibiotic, sulfinemycin, in additon to antibiotic A204A. This paper describes the fermenation conditions, the isolation procedures, and the chemical structure of this new anthelmintic antibiotic which contains a trans ot,/3 unsaturated thioamide S-oxide group which has not been previously observed in natural products.