The temperature dependence for the rate of reconstitution or polymerization ( k +) at neutral pH of the protein, flagellin, to flagella was measured using Ostwald-type viscometers. Similarly, the kinetics for the reverse process, the thermally-induced depolymerization of flagella filaments to the flagellin monomer ( k −) was measured. The temperature at which k − equals zero was used to define the thermal dissociation temperature or melting point of flagella filaments. The remarkable similarity of melting points obtained (36.8 ± 0.2 deg. C) for flagella isolated from three Salmonella strains (SJ670, SJ25 and SJ30 bearing H-antigen types i, 1.2 and e, n, x, respectively) suggests that the structural stability of these different protein filaments is also similar. On increasing the temperature between 12 and 28°C, k + increased smoothly and had a Q 10 of 1.8. Above 28.0, k + decreased rapidly and fell to zero at a temperature near 37°C, its precise value varying with the bacterial strain. This result supports the prior hypothesis (Gerber & Noguchi, 1967) that on heating, a reversible co-operative transconformation occurs between different states of the protein; in one state, flagellin (M) can polymerize to flagella, whereas its conformational isomer(s) may do so with difficulty or not at all. For strains SJ25 and SJ30 the rates of polymerization and depolymerization both fall to zero near 37°C. Therefore, mixtures of monomer and flagella fragments (short polymers or “seeds”), in all ratios, appear to be in equilibrium at temperatures near this critical temperature, and neither polymerization of flagellin to flagella nor melting of polymers is apparent. Measurements made on flagella from strain SJ670 showed that k + and k − approached zero at 45 and 37°C, respectively. Within this temperature range the conc entration of monomer in equilibrium with filaments was determined. By a null -point type experiment, solutions of monomer and seed were mixed to find the ratio that showed neither increases (polymerization) nor decreases (depolyme rization) in viscosity with time. An unexpected finding was that the temperature defines a critical monomer concentration, which exists in equilibrium with any concentration of filaments (and not the ratio of monomer-to-filament concentrations). Thus, the polymerization of fiagellin to flagella corresponds to a phase change akin to either crystallization or condensation. Application, of the Clapeyron-Clausius equation to the results obtained yields a heat of condensation of 70 kcal/mol of monomeric protein. The enthalpy change associated with M ⇌ M i is estimated as 110 kcal/mol of protein. Since the heat content of these various forms of flagella protein lies in the order M i > F > M, by difference we estimate the enthalpy change for the conversion of monomers to polymers to be 40 kcal/mol of monomer.