1. After the transference of fresh water mussels (Anodonta cygnea) from 4 to 20-24°C, the heat resistance of ciliary activity in gills is increased significantly in 1 day and then further in 1-4 days to a new steady level.2. If gills isolated from animals at 4°C are transferred to 24°C, the heat resistance of ciliary activity increases in 18-24 h to a level steady for 3 days. This level is equal to that found in gills of whole mussels after 1 day at 20-24°C.3. The response of isolated gills is not due to isolation, but probably to heat shock.4. The transfer of isolated gills to the higher temperature induces in 24 h the synthesis of 90 kDa stress protein, but does not affect the cell membrane fluidity.5. The heat shock response occurs also in isolated gills, but the thermal resistance acclimation of ciliary activity only occurs in the gills of whole mussels.