Bacterial strains CF-20 (CECT5719) and C-148 (CECT5718), isolated from cultures of larvae of molluscs, are shown to produce substances 1-5 with strong antibiotic activity against Vibrio anguillarum (MIC: 0.03-0.07 mug/mL) and identified as the dd-diketopiperazines cyclo(d)-Pro-(d)-Phe (1), cyclo(d)-Pro-(d)-Leu (2), cyclo(d)-Pro-(d)-Val (3), cyclo(d)-Pro-(d)-Ile (4), and cyclo-trans-4-OH-(d)-Pro-(d)-Phe (5). Comparison with other stereoisomers indicates that inhibition of V. anguillarum is associated with the presence of at least one d-amino acid in the diketopiperazine system. This is the first time a series of dd-diketopiperazines has been isolated from a single natural source and their inhibitory activity against V. anguillarum described.