The finding of intact doublets of Unionoids in Belt Schutsloot in November 2004 reminded me of the massive death of Unionoids in the summer 2003. In this summer a high rate of Unionoids died after several weeks with high temperature. It is was very remarkable that it concerned mostly Unionoids older than 3-4 years. The younger one’s, and even other species of molluscs seemed to be unharmed. What caused their death?