1. The relation of temperature to the pedal rhythm of Balanus balanoides L. has been studied under otherwise constant conditions.
2. The frequency of movement increases with temperature, showing three groups of thermal increments and three critical temperatures. Five animals yielded µ = 5,700 above 14.5° C. and 12,100 below; 3 gave µ = 7,800 above 9.3° and 22,500 below; while 9 showed µ = 9,500 above 8.1° and 22,100 below.
3. The upper critical temperatures, above which different effects appeared in different animals were 23.4°, 26.0°, and 27.0°. Above 27.0° none of the valves remained open.
4. Excepting the values 5,700 and 9,500, the increments are similar to those previously found to be associated with respiratory and with neuromuscular activities.
5. Dilution of the sea water with from 3 to 4 per cent fresh water decreases the rate without altering the increments. More than 4 per cent dilution causes irregularity.