Mussels from one coast- and one fjord location in the Sognefjord were sampled (depth 3-5m) every other week during the period March-November 1997. Yessotoxin (YTX) was analysed by HPLC using the fluorogenic reagent DMEQ-TAD for derivatizing of YTX. The mouse bioassay for the diarrhetic shellfish poisoning (DSP) toxins was performed essentially according to the Japanese method with slight modifications including chloroform in the final step of extraction. In addition to okadaic acid and DTX-toxins chloroform extraction includes a wide spectrum of toxins, including pectenotoxins, yessotoxins and among others unknown toxin(s) with neurotoxic potential. The concentration of YTX in mussels determined by HPLC analysis was on average 3 times higher at the location deep in the fjord compared with the coast location. A maximum toxin concentration was observed in May in mussels sampled at the fjord location. The mouse bioassay of most mussel extracts from both fjord and coast location showed high toxicities from April and throughout the sampling period. Most of the samples analysed contained levels of YTX which alone could contribute to the high mouse toxicity of the chloroform extracts. At YTX doses of about 560µg /kg body weight or above, the survival times were in the narrow range of 40-50 minutes. The very short survival times (<30min.) in the mouse bioassay of many of the extracts from the fjord location indicate that the mussels also might contain unknown toxin(s). When survival times were very short, the mice showed PSP-like symptoms, with jumping in the early stages followed by respiratory arrest. Survival times shorter than 40 minutes have not been observed for YTX upon i.p. administration to mice.