<jats:title>Abstract</jats:title><jats:p>In the culture broth of the marine <jats:italic>Streptomyces</jats:italic> sp. B 9173 the two novel diketopiperazines maremycin A (<jats:bold>1</jats:bold>) and B (<jats:bold>2</jats:bold>) were detected by chemical screening. Their structures were elucidated and confirmed by the synthesis of the oxindole derivative <jats:bold>4</jats:bold>.