Body temperature distributions of ectotherms which are free to select any environmental temperature throughout a wide range usually are negatively skewed, that is, a wider range of selected temperature falls below the median than above it. This negative skewness can be understood as the consequence of the regulation of a temperaturedependent rate process whose rate, as an exponential function of temperature, is maintained within a normal curve of error. Analysis of frequency distributions of body temperature show that they can be characterized by the median and two additional parameters. One of these can be used to approximate the Q10 of the hypothetical rate process. Mathematical and graphical techniques for the description of body temperature distributions are developed, and are proposed as useful tools for comparative and experimental studies of body temperature regulation.