Pseudochattonella farcimen (Eikrem, Edvardsen, et Throndsen) is an ichthyotoxic alga within the Dictyochophyceae (Heterokonta), which has been shown to form blooms in Scandinavian waters every year since 1998. To improve our understanding of the biology of this alga and to facilitate future genomic studies, we report the sequencing and analysis of >10,000 expressed sequence tags (ESTs) corresponding to 8149 gene models from this species. A direct comparison with EST libraries from other heterokonts revealed several functional categories to be significantly overrepresented among the P. facimen ESTs, such as genes involved in cell communication, transporters, or genes targeted to cell organelles. Interestingly, P. farcimen ESTs also code for a high proportion (1.4%) of proteins related to fatty acid metabolism, including eight fatty acid desaturases and two phospholipase A2 genes. Three of the desaturases belong to a family of delta-4 desaturases, known so far only from haptophytes, where they catalyze the conversion of n3-docosapentaenoic (n3-DPA) acid to docosahexaenoic acid (DHA). These findings may partially explain the unusual fatty acid profiles observed in P. farcimen and are discussed both from an evolutionary point of view and in relation the ichthyotoxic effects of this alga