The temp. coefficients for the amylases of human saliva, fish and terrapin pancreas were determined for the range between 3.5[degree] and 63[degree] C; measurements were made at 6 temps., using 2 iodine and 1 Cu reduction methods simultaneously. Under comparable conditions the optimal temps. of the amylases were, human > terrapin > fish. Fish amylase acted more efficaciously at 3.5[degree] C than did terrapin and human amylases; the order was. fish > human > terrapin. Certain factors were found to influence the so-called optimal temp. The enzyme-substrate ratio had an effect; the larger the proportionate amount of enzyme, the higher the optimal temp. for a given period. Also the longer the period of digestion, the lower the optimal temp. Both these effects depended on the progressive heat inactivation of enzyme at the temps. investigated.