Since 1962, annual surveys on the mytilotoxin titre in mussels from Oslofjorden have been carried out. Samples from other localities have occasionally been incorporated in the monitoring programme, Great annual variations in the mytilotoxin ntre were observed, rangmg from below 200 MU/100g to 5479 MU/100 g of mussel meat. The highest titre ever observed in Norway, 17400 MU/100 15, was recorded. in. 197.9 in samples from Nordasvannet (Bergen), In Oslofjorden, the mytilotoxin titre in mussels appears to be correlated With the bloom of certain dinoflagellates. The highest tit res were usually recorded during early June.