Abstract
The freshwater musselDreissena polymorpha Pallas was sighted for the first time in Lake Zurich about 1969. This meant a real problem for the water treatment of lake water. By means of a pump and vertical net hauls it was tried to add more information to the scarce results on Lake Zurich. Additional dates about temperature, depth of Secchi disk visibility and beam transmittance were collected. There was a first appearance of the larvae ofD. polymorpha at the beginning of June. Following the mean temperature of the epilimnion the number of individuals/m2 reached max. 210,000. The greatest concentration measured in 1974 of larvae/m3 was 54,375 at a depth of 4 m. The last appearance ofD. polymorpha was observed at the end of October.
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