The production of violacein by Pseudoalteromonas sp. 520P1 has many features of quorum sensing. Signaling molecules were extracted from bacterial culture and subsequently identified as N-(3-oxooctanoyl)-homoserine lactone and N-tetradecanoyl-homoserine lactone. The former but not the latter induced the production of violacein in strain 520P1. We conclude that N-(3-oxooctanoyl)-homoserine lactone is a signaling molecule involved in the production of violacein.