The stability to denaturation by heat and guanidine hydrochloride of seven vertebrate (including skeletal, cardiac and smooth muscle) tropomyosins and three invertebrate tropomyosins was examined. The transition profiles were discontinuous and in many cases distinct plateaux were observed which indicated the presence of unique partially unfolded states at intermediate temperatures and guanidine hydrochloride concentrations. The denaturation by guanidine hydrochloride could be described in the majority of cases by a model in which the native state unfolds to a partially unfolded stable intermediate which then unfolds to the completely denatured state. On this basis it was possible to estimate the free energies of unfolding in water. It was shown that part of the IX-helical structure of tropomyosin is only marginally stable and the free energy of unfolding in water of this segment is less than values found for globular proteins, whereas another segment (or segments) has a stability comparable to that found for globular proteins. The stepwise unfolding may be explained in terms of the coiled-coil interactions in tropomyosin.