Measurements of birefringence, ultraviolet dichorism and quasielastic light scattering were carried out on F-actin in solution and on the thin filaments of glycerinated myofibrils. The birefringence of the I-bands of myofibrils was of the same order of magnitude as that of F-actin or the F-actin-tropomyosin-troponin complex oriented in vitro at the same concentration. The ultraviolet dichroism spectrum of the I-bands was very similar to that of F-actin or the F-actin complex in vitro, which is due to orientation of bound ADP and tryptophan residues in F-actin. Quasielastic light scattering measurements, electronmicroscopic observations and the analyses of the electro-optic effect of the I-bands suggested approximately the same flexibility for F-actin in vitro and for the thin filaments in vivo. These optical measurements which were made under various conditions provide evidence for a conformational change induced by calcium ions in F-actin both in vivo and in vitro. This conformational change was found to be amplified by the interaction of F-actin with myosin. This is a brief review of our investigation on the dynamics of F-actin and the thin filament in vivo and in vitro by optical methods.