1. Slow and fast contractions, as well as the corresponding excitatory junctional potentials (ejp's), were recorded from the closer muscle of walking legs subjected to temperatures ranging from 10 to 35 °C. 2. Maximal ejp amplitudes and tension occurred at temperatures between 22° and 28°. The temperature of the animals in the field was between 26° and 27.5 °C. 3. When the temperature was raised above 27° tension development and ejp amplitude declined. At 31° the values of both were usually less than half those recorded at 27°, and they declined to zero when the temperature was raised above 34.5°. 4. At temperatures below 22° tension development and ejp amplitude declined also. The half-time of the decay of ejp's increased with decreasing temperature, causing summation of ejp's at low frequencies. 5. The membrane potential of the muscle fibers increased with increasing temperature above 12 °C by about 1 mV/l°. 6. The running speed of intact animals tested in the field decreased when body temperature was lowered below the ambient temperature. The typical flight response could not be elicited when animals were cooled by only 3°, to 23°. Rewarming to 26° restored normal behavior. Animals cooled to 10° or less were incapable of any movement, even when prodded. They usually recovered within a few minutes after being warmed to 26°. 7. The observed temperature effects are discussed in terms of altered transmitter output and transmitter re-uptake by nerve terminals and in terms of the significance of changes in synaptic efficiency and conduction time for the neural mechanisms underlying behavior. The results suggest several mechanisms by which temperature effects on nerve-muscle systems might be compensated in crustaceans adapted to more widely fluctuating temperatures.