Oxygen consumption and Zn65 excretion were measured at temperatures of 17°C and 27°C for two similar—sized species of wood—feeding insects, Cryptocercus punctulatus and Popilius disjunctus. In each species, rate of oxygen consumption was approximately doubled, and ZN65 excretion was significantly accelerated at the higher temperature. In animals held at a given temperature, @m1 O2 consumed/g live body weight was similar. However, radiozinc excretion was much more rapid in P. disjunctus than in C. punctulatus at each temperature. The species differences in Zn65 excretion may reflect the extreme differences in feeding habits of these otherwise ecologically similar inhabitants of decaying logs. Excretion rate of the radionuclide Zn65 seems to be a good index of metabolic activity and may be of promise in the study of trophic relationships.