ABSTRACT: Planktivorous fish are key potential vectors for the phycotoxin domoic acid (DA), produced naturally by diatoms from the genus Pseudo-nitzschia. The diet of the Atlantic sardine Sardina pilchardus is largely dominated in number bymicroplanktonic species such as chain-forming diatoms, making the accumulation of this toxin and its transfer to the higher trophic levels likely. DA concentration in sardine tissues and Pseudo-nitzschia ingestion were monitored fortnightly during2002 and 2003 off the NW coast of Portugal, where seasonal upwelling events are responsible for the occurrence of algal blooms. Sardine stomach content analysis showed that Pseudo-nitzschia prey reached concentrations as high as 7.8 ×10⁶ cells g^-1; in some cases this diatom genus represented more than 99% of the phytoplanktonic prey identified in the stomachs. Four different diatom species were distinguished using scanning electron microscopy (SEM): P.australis, P. pungens, P. pseudodelicatissima and P. delicatissima. DA accumulation in sardines was linearly dependent on P. australis consumption. Toxin content per individual cell was estimated by comparing DA and P. australisconcentrations in the stomach contents. DA production by P. australis was significantly higher in the summer months than during the spring. In both years, DA in sardine guts was initially detected in May and peaked several times until late summer.Toxin distribution in the different tissues was also determined, with the highest DA levels detected in the intestine. The maximum toxin concentration observed in sardine guts was 128.5 µg DA g^-1. No DA was found in the sardinemuscle; consequently implications for human health appear minimal.