In order to test the range of tolerance and verify the preferred optimum temperature of the five-lined skink, an experimental terrarium was set up providing extremes of temperature at each end. A false floor of 1/8 inch wire screen was provided, with a seven-inch strip of galvanized sheet metal beneath it at each end. Beneath the screen and sheet metal at one end the space was filled with chopped ice, and "dry ice." Observations were made on hot, clear summer days, with the terrarium arranged so that the half of it containing ice, was in shadow, and the other half was in sunshine. The strip of metal, warmed by direct sunlight, became uncomfortably hot to the touch while at the other end the sheet metal and overlying screen were cooled by the ice. A narrow zone across the middle of the terrarium had screen but no underlying sheet metal and was the only part within which the lizard could maintain normal temperature, one end being uncomfortably hot and the other end too cool. A large dead skink left on the metal strip in direct sunlight for five minutes had a cloacal temperature of 45.3°C., and after five minutes on the screen at the cool end, its temperature had dropped to 25.5°C. On several occasions a number of skinks were put in the terrarium and their temperatures taken at brief intervals. Temperatures ranged from 21.6°C. to 37.7°C. but were mostly within a much narrower range, from 28° to 36°C. One skink that seemed to be sick was sluggish in behavior, not responding to the extremes of temperatures as readily as the other individuals and his temperature fluctuated widely and irregularly. Eliminating this individual, 66 temperature readings taken, from five other skinks, gave a mean of 32.6°C. ± .235. While nearly all the temperature readings were within a range of ten degrees, two of the readings were outstandingly low and perhaps should be discarded. If this is done, a mean of 33.8°C. ± .19 is obtained for the remaining 64. There is distinct bimodality in this series however, with a mean of 34.2° for the 49 higher readings, and a mean of 28.8°C. for the 15 lower temperatures. A similar bimodality is evident in the readings obtained from skinks caught in the open under natural conditions. It seems that the lower readings result from lags in the skinks' response when body temperature drops slightly below the optimum. The skink is quick to make adjustment whenever its temperature appreciably exceeds this optimum level, and is in extreme discomfort at only a few degrees higher temperature. At slightly lower temperatures, however, the skink experiences no discomfort, and only slightly decreased efficiency in its various functions, and its thermoregulatory behavior in making readjustment toward the optimum is likely to be leisurely and interrupted unless its temperature drops below 28°C.