1. Apparatus is described for the irradiation of Daphnia magna by 3 MHz. ultrasound, under controlled temperature conditions.

2. The lethal level for heat exposure was established for Daphnia.

3. It is shown that there was a threshold of irradiation level below which immediate death was due to heat.

4. Above the threshold level for non-thermal damage, streaming in the environment of the animal became considerable.

5. Long-term observations confirmed the existence of a threshold level for non-thermal damage during irradiation.

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