In our screening research for plant growth regulators from microbial origins, we found that a culture filtrate of Streptomyces sp. strain 3C02 inhibited the growth of lettuce seedlings. The structure of the active compound was determined by NMR, FAB mass spectral and amino acid analyses, and it was named rotihibin A (1). In this communication, we report the isolation and structural elucidation of rotihibin A (1). Rotihibin A (1) suppressed the root growth of lettuce seedlings at a dose of 1-2 ppm, but was not lethal even at 150 ppm. Furthermore, the plants recovered their normal growth when they were removed from the medium containing 1. Other biological activities of 1 are now under investigation.