Scattering curves were obtained for a-tropomyosin two-stranded, coiled coils having the helix fraction ranging from 0.40 to 0.95 by small angle X-ray scattering with synchrotron orbital radiation. The observed curves were analyzed by comparison with the theoretical scattering function computed for two chain-conformational models : In model I, it is assumed that both ends of the helical protein unfold (fray) first as temperature is increased; in model II, it is assumed that the middle part unfolds first. The resulting theoretical functions are very different for the two models. Comparison with the observed curves, using a reasonable value for the effective bond length in the randomly-coiled state (12 Angstrom), clearly shows that model I successfully mimicks the experiment whereas model II does not. This finding answers a long-standing question as to the location of the unfolded region in the tropomyosin molecule under conditions when it is still mostly dimeric and only somewhat unfolded.