In 2002 several hundred people were taken ill after eating self-harvested brown crabs (Cancer pagurus) in the southern part of Norway. The symptoms were similar to diarrhetic shellfish poisoning (DSP) although with a somewhat delayed onset. This happened at the same time as an unusual early bloom of Dinophysis acuta had lead to high amounts of DSP toxins in blue mussels (Mytilus edulis) in the same area. The proposed cause of the intoxication was that crabs had accumulated toxins by eating blue mussels. Analyses of crab material from the area revealed very little free toxin in the form of okadaic acid (OA). However, after alkaline hydrolysis of the material, the amounts of OA found in the crabs were above the toxic level. MS/MS analysis of a sample from one intoxication episode indicated presence of the 14:0, 16:1, 16:0 and 18:1 fatty acid esters of okadaic acid. Esterified OA constituted more than 90% of total identified DSP toxins in crabs, indicating that not only esterified toxin from mussels was accumulated, but also that appreciable transfer of OA to OA-esters occurred in the crabs.