Okadaic acid (OA) is a relatively stable polyether toxin causing diarrhetic shellfish poisoning, DSP. OA is produced by dinoflagellates such as Dinophysis acuminata and D. norvegica, which are common throughout the Baltic Sea. This protein phosphatase (PP) inhibitor can induce toxic effects such as hepatocyte apoptosis after in vitro exposure. The aim of this study was to analyse the liver and muscle of flounder from the Baltic Sea for okadaic acid. 30–50 flounder (Platichthys flesus) were caught at depths between 6 and 20 meter in different areas of the northern Baltic Sea every 3–4 weeks in June– September 1996–1998. The presence of OA was analysed using HPLC. The flounder caught in August 1996 contained OA (222 ng OA g-1ww, liver tissue). This was the first observation of OA in fish from the northern Baltic Sea.