One-step immunochromatographic assay using colloidal gold-labeled monoclonal antibody (Mab) probe for the rapid detection of brevetoxins (PbTxs) in fishery product samples was developed. The described assay was based on a competitive format using two antibodies. The primary antibody was conjugated with colloidal gold (detector reagent), the secondary antibody (capture reagent) was immobilized within a defined detection zone (control line) on a diagnostic cellulose nitrate membrane. The toxin in sample compete with immobilized toxin to bind with gold conjugated Mab. The mobile complex (colloidal gold-Mab-toxin) can be captured by the secondary antibody but cannot be captured by BSA-PbTx (test line). The color density of the test line correlated with the concentration of PbTx in sample in the range 10-4000 ng mL(-1). Spiked samples were detected by the assay and the visual detection limit was found to be 20 ng mL(-1). This qualitative test based on the visual evaluation of results did not require any equipment. The assay time for PbTx detection was less than 10 min, suitable for rapid testing on-site.