Since 1987, brown ring disease has caused mass mortalities on various cultured clam-beds. The causal agent is a Vibrio sp. called Vibrio P1. The problem is to perform a suitable, reliable, rapid and sensitive technique to detect this bacterium in the environment. Therefore, monoclonal antibodies (mAb) were prepared according to hybridoma technology. One mAb was selected by enzyme-linked immunosorbent assay (ELISA) because of its capacity to react specifically with V. P1. Nitro-cellulose membrane immunoassay (colony-blot ELISA) was developed to detect directly V P1 in samples. Results of identification of 223 bacteria by the biochemical method and by the colony- blot ELISA method were compared. The specificity and the sensitivity were 100% in both cases. Our results indicate that the colony-blot ELISA is sensitive, easy to perform and is especially suited for routine diagnostic work.