1. Cold hardiness to the intensity factor of low temperature can be measured by moisture content, undercooling point, and blood conductivity.2. Up to the time when a given insect can survive freezing, undercooling is a reliable measure of cold hardiness. Beyond the point when an insect can survive freezing, undercooling measures but a part of the total cold resistance of a given insect.3. Conductivity measurements are found proportional to cold hardiness throughout the whole year. In some insects there is insufficient free body fluid in winter on which to determine blood conductivity.4. For each species there is a different set of physical constants which measure the cold hardiness of that species.