The behavior of individuals of the genus Peromyscus has been studied in an 8 ft. aluminum tube in which a gradient of temperature could be established. When the tube was at a uniform temperature the mice were found most commonly at the two ends. With the gradient set up providing a range of temperatures from 6° to 50 °C. no mice were found at the hot end, the tendency to remain at the cold end was greatly reduced, and a broad peak in the distribution appeared between 20° and 30 °C. This combined distribution suggests a much less precise selection of temperature than was characteristic of individual mice. The modes of the individual distributions varied from 20° to 28 °C. thus accounting for the relative flatness of the combined distribution. The mode of the combined distribution was 24 °C. A few experiments were done (1) with movement of air through the tube (2) in shorter gradients and (3) in a vertical gradient all of which point to the conclusion that it is the temperature of the substratum on which the animal walks that determines its response.