Results from a study of Thermal Flicker-Fusion (TFF) are reported. The study tested the effect of input factors similar to those significant in varying CFF (Critical Flicker-Fusion) thresholds for vision. More specifically, amount of area stimulated, intensity level and PCF (Pulse-to-Cycle Fraction) were tested separately, with other factors held constant. Thermal Flicker-Fusion (TFF) thresholds were obtained by means of intermittent radiant energy produced by a modified Hardy- Wolff-GoodeH do!orimeter. Four trained observers were employed, and were tested under all conditions replicated five times. Stimulation was applied to the two body areas (upper back and volar forearm).In general, the thermal fusion results show close correspondence to the functions produced by varying similar parameters in the visual modality. In both, curves relating sensory fusion thresholds to PCF increase to a maximum and then decline. Significant curvi1inearity is seen. Also, increases in amount of area stimulated produces a corresponding increase in both types of fusion thresholds, i.e., results show a significant linear relation between TFF and the variable of area. One major discrepancy does occur, however. In the visual modality, fusion curves produced by variation of energy level increase to a maximum and then decline. The thermal fusion curves ascend with increases in intensity, as one might expect, but no decline occurs in the range tested. Results indicate a significant linear relation between intensity and TFF. Even though "ascending" judgments were analyzed separately from "descending" judgments (due to the resulting absolute differences in rate between the two types of judgments), the form of the curves remained similar. Moreover, the form of the functions is also similar for the different body areas tested, although differences in absolute rates at fusion are noted. Results are compared to previous fusion studies in the thermal and other modalities. Decreases in inter-0_ variability were seen with increasing practice. Results of the present study were similar to those of Kastorf's investigation, in that the form of the analogous curves tended to correspond. Following this, a theoretical explanation in terms of A1ternation-of-Response theory is suggested. The significance of the thermal experiments to von Bekesy's hypothesis relating thresholds and density of innervation are briefly discussed.