In December 1997, 56 people reported gastrointestinal symptoms of either vomiting or diarrhoea after ingestion of pipis (Plebidonax sp. ). This bivalve mollusc is collected from the intertidal zone of sand on gently sloping ocean beaches, a few centimetres below the surface. Most commercial supplies come from Ballina, New South Wales, which has an annual collection of 300,000 kilograms. Two frozen samples of implicated pipis collected from cases were tested for the presence of toxins. ELISA tests for DSP toxins proved negative, but mouse bioassays were positive, indicating the presence of a lipophilic toxin. Liquid chromatography-mass spectrometry (LC-MS) analyses indicated low levels of okadaic acid, but they were insufficient to account for illnesses. However, other signals were observed that indicated the presence of two possible polyether-like compounds, all with a molecular weight of 876.5, and these were absent in non-toxic samples. Preparative chromatography was performed to isolate these compounds and structural information was generated using tandem and high resolution mass spectrometry and NMR spectroscopy. These compounds were identified as pectenotoxin-2 seco acids, toxins that were reported recently in New Zealand samples of Dinophysis acuta and greenshell mussels, although not associated with human illnesses. Examination of toxic samples of pipis revealed the presence of Dinophysis acuminata and D. tripos.. As LC-MS methods were further developed, a number of related compounds were observed, and these are believed to be metabolites or degradation products of the main compounds.