Several investigators have documented that the marine phytoplankter Phaeocystis pouchetii produce and excrete some compound that has adverse effects on its surroundings, but the chemical composition and structure of the active agent has so far been unknown. In the present study we used mass spectrometry to investigate the structural properties of the putative toxin. Colonial cells of P. pouchetii were collected along the coast of northern Norway and cultivated in the lab for a limited period of time prior to harvesting by filtration. Harvested cells and culture filtrate were extracted separately with organic solvents, and a yeast cell bioassay was used to track the toxic fractions during extraction and purification with HPLC. We found the organic extract from the culture filtrate to be toxic, and after purification with RP-HPLC the cytotoxic activity was recovered as one fraction. When the toxic fractions were pooled and analysed by GC-MS we were able to identify 2-trans-4-trans-decadienal by comparing retention time and fragmentation pattern to a commercial standard. This is the first report of a polyunsaturated aldehyde produced by a marine alga belonging to the class Haptophyceae, and this implies that production and release of these reactive compounds are not limited to diatoms.