SUMMARY: Comparison of pectenotoxin (PTX) profiles between the toxic dinoflagellate Dinophysis acuta and scallops Pecten novaezelandiae collected at Wedge Point, Queen Charlotte Sound, New Zealand was carried out by liquid chromatography–mass spectrometry (LC-MS) with turbo-ionspray ionization. Although the major PTX homolog in D. acuta was pectenotoxin-2 (PTX2), the scallops contained pectenotoxin-2 seco acid (PTX2SA) as the predominant toxin. Pectenotoxin-2 isolated from D. acuta was rapidly converted to PTX2SA and its epimer 7-epi-pectenotoxin-2 seco acid (7-epi-PTX2SA) in the scallop extracts. These results indicate that PTX2SA and 7-epi-PTX2SA arose from the conversion of PTX2 by scallop tissue. The results indicate that New Zealand scallops have an ability to reduce the cytotoxicity of PTX2 by conversion to PTX2SA.