A microtitration plate, antibody capture, enzyme-linked immunosorbent assay was developed for the detection of Aeromonas hydrophila serotype O: 11 (highly virulent strains). The assay utilizes a detector antibody which shows no cross-reactions with Aeromonas strains other than serotype O: 11 or non-Aeromonas competing organisms. The detector antibody is mixed with the sample and incubated for 1 h, microcentrifuged and the supernatant fluid (unadsorbed antibody) titred in a microtitre plate coated with A. hydrophila cells from serotype O: 11. All the A. hydrophila strains from serotype O: 11 tested reacted strongly with the detector antibody. Also by culturing and performing the immunoassay with the detector antibody we established and quantified the presence of A. hydrophila O: 11 in different foods.