Comparison of pectenotoxin (PTX) profiles of toxic dinoflagellate Dinophysis acuta, Greenshell mussels (Perna canaliculus) and Blue mussels (Mytilus galloprovincialis) collected from Wedge Point, Queen Charlotte Sound, New Zealand was carried out by liquid chromatography-mass spectrometry with turbo-ionspray ionization. Although the major PTX homologue in D. acuta was pectenotoxin-2 (PTX2), both Greenshell and Blue mussels contained pectenotoxin-2 seco acid (PTX2SA) as the predominant toxin. More than 90% of PTX2 isolated from D. acuta was rapidly converted to PTX2SA and its epimer 7-epi-pectenotoxin-2 seco acid (7-epi-PTX2SA) in the Greenshell mussel extracts. The conversion from PTX2 to PTX2SA and 7-epi-PTX2SA was not observed in phosphate buffers at various pH ranging from 4.1 to 9.1. These findings indicate that PTX2SA and 7-epi-PTX2SA are not artifact toxins resulting from hydrolysis of PTX2, but arise from the conversion of PTX2 by mussel tissues.