We have obtained clear evidence for the flexibility of native scallop adductor thin filaments by studying the temperature and ionic strength dependence of the average decay constants obtained from intensity fluctuation spectroscopic (IFS) measurements. The low-angle (10-25 degrees ), average decay constants obtained from time autocorrelation functions of scattered light were independent of concentration (0.08-1.3 mg/ml), scaled with the ratio of temperature to solvent viscosity, T/eta, over a range of 4-45 degrees C, and yielded a value for the translational diffusion coefficient of D(T) (5 degrees C) = (1.24 +/- 0.06) x 10(-8) cm(2)/s. From this value and the Broersma relation for rigid rods, we find an average filament length of 1.06 +/- 0.06 mum. Quantitative sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that at high temperatures (> 35 degrees C) or in 0.6 M NaCl, tropomyosin completely dissociates from native thin filaments. Decay constants from high-angle (60-150 degrees C) IFS temperature dependence measurements do not scale with T/eta and hence do not show the temperature dependence expected for rigid rods. The differences are not due to any change in length distribution of filaments with temperature or to the free tropomyosin in solution, but are attributed to nonrigid motions of the filaments. Similar experiments on samples in high- and low-salt solvents gave results consistent with this interpretation.