The purpose of the present study was to examine the extent of the variations in mean skin temperatures by Hardy and DuBois 7-point formula when different single trunk sites were assigned (T _sk-chest, T _{sk- abdomen}, and T _{sk-upper back} for the chest, abdomen and back, respectively). The average of the three trunk sites (T _sk-all) was considered as a reference value. Eight male subjects underwent twelve experimental conditions: activities (rest and exercise) × clothing (Control, Tyvek and Vinyl condition) × air temperatures (25°C and 32°C). The results showed that 1) for rest conditions, there was no difference among T _sk-chest, T _{sk- abdomen}, T _{sk-upper back}, and T _sk-all; 2) for exercising conditions at 25°C, T _{sk-upper back} tended to be greater than T _sk-chest, and T _sk-abdomen. The differences were statistically significant for Control and Tyvek conditions (p<0.05); 3) for Control during exercise, T _sk-abdomen was significantly lower than T _{sk-upper back} (p<0.05); 4) T _sk-all were not statistically distinguished from T _sk-chest, T _sk-abdomen, and T _{sk-upper back}; 5) for Control during exercise, T _{sk- upper back} overestimated T _sk-chest and T _sk-abdomen at T _a 25°C, while T _sk-abdomen underestimated T _{sk-upper back} at T _a 32°C. The variations were attributable to the differences in thermal dynamics of the abdomen and upper back tissues during exercise. For estimating the T _sk of lightly clothed male subjects during exercise using Hardy and DuBois 7-point formula, reliance on the abdomen or the upper back as a site representative of the trunk is not recommended, but the average of the chest, abdomen, and back temperatures is recommended as the trunk temperature. In the case that the minimum number of skin sites is required due to dynamic exercise, a preferred single site on the trunk was the chest for male subjects.