Piericidin A1, 3'-rhamnopiericidin A1, and a novel compound piericidin E, a new quorum-sensing (QS) inhibitor against Chromobacterium violaceum CV026, were isolated from the culture broth of Streptomyces sp. QS is well known as a microbial signaling system and controls certain types of gene expression resulting in bioluminescence, biofilm formation, swarming motility, antibiotic biosynthesis, and virulence factor production. C. violaceum CV026 is commonly used to determine qualitative and quantitative QS activity. The structures of piericidin derivatives were characterized, and their QS activities were determined.