Filamentous actin (F-actin), which occurs abundantly and ubiquitously in living mechanisms such as muscle, nonmuscle cells, and cytoskeleton, exhibits a liquid crystal polymorphism as its concentration is increased. The double-stranded, helical F-actin filaments are 1-70 mu m long in our samples. The chirality of the filaments is consistent with the experimental observation of the characteristic cholesteric fingerprint and uniform textures. In addition to the chiral nematic phase, our optical microscopic studies show the occurrence of tilted chiral smectic phases, e.g., smectic C*, I*, or F*, and a smectic B phase. Measurement of optical rotatory dispersion in the cholesteric phase confirms the prediction of the de Vries equation describing the optical rotatory dispersion behavior for cholesterics. The observed lyotropic polymorphism is consistent with the Flory lattice model and a semiflexible polymer mesogen model of F-actin.