An enzyme-linked immunosorbent assay (ELISA) has been developed which detects Vibrio parahaemolyticus biotype alginolyticus. This sandwich ELISA, amplified using avidin-biotin, has a sensitivity threshold of 1-103 bacteria per ml (102 bacteria per well) and appears to be specific for V. alginolyticus. Levels of bacteria in the haemolymph of penaeid shrimps challenged with V. alginolyticus were estimated using ELISA and the plate-count method.