(1) The effect of temperature on the viscosity of the protoplasm has been determined in the unripe eggs of Nereis diversicolor by the centrifuge method.

The viscosity rises as the temperature falls, the rise becoming rapid near 0°C.

(2) These changes of viscosity with temperature are similar to those described by Weber (21) in the protoplasm of certain plant cells.

(3) Attention is drawn to the fact that the temperature coefficients of biological processes should be corrected for the viscosity of the protoplasm in which they occur.

(4) It is shown that the relative changes of protoplasmic viscosity with temperature are probably of the order required to make the temperature coefficients of biological processes constant when corrected for the viscosity factor.

(5) The magnitude of the temperature coefficients of biological processes when corrected for viscosity changes is probably of the same order as that of the temperature coefficients of a chemical process when corrected for changes in the viscosity of its medium.