Abstract 1. 1. Lateral ciliary activity of the gill of Mytilus edulis was studied with respect to temperature changes and acclimation. 2. 2. The Q 10 varied with the temperature range. 3. 3. Maintaining animals for 2 months at 18°C, as compared to 6°C, shifted the responses of the ciliated cells and resulted in acclimation. 4. 4. The study located the site of the temperature sensitive mechanism in the lateral cells of the gill, presumably in the lateral cells themselves.