Aims: To investigate the antimicrobial spectrum of Lactococcus piscium CNCM I-4031 and its protective effect in cooked and peeled shrimp against Brochothrix thermosphacta. Methods and Results: Sixteen pathogenic and spoiling bacteria were inhibited in Elliker, but not in shrimp juice agar plates. In shrimp packed under modified atmosphere and stored at 8 degrees C, B. thermosphacta (103 CFU g-1) was inhibited by 4 center dot 1 log CFU g-1 when co-inoculated with L. piscium (106 CFU g-1). Brochothrix thermosphacta spoiled the product after 11 days, with the emission of strong butter/caramel off-odours. In co-culture with L. piscium, sensory shelf-life was extended by at least 10 days. The inhibition was partially explained by a drop in pH from 6 center dot 6 to 5 center dot 6. The physicochemical composition of shrimp and shrimp juice was established to identify the inhibition mechanisms involved. Conclusion: Lactococcus piscium CNCM I-4031 has a wide antimicrobial spectrum. The strain inhibits B. thermosphacta in shrimp and significantly prolongs sensory shelf-life. Significance and Impact of the Study: Lactococcus piscium CNCM I-4031 is shown to be a promising agent for improving shrimp quality and may be tested against pathogens and in other food matrices. Knowledge of the physicochemical composition of shrimp and shrimp juice will allow the development of a chemically defined model medium for determining the inhibition mechanisms involved.