AbstractThe total strain birefringence of F‐actin isolated from chicken gizzards was measured as a function of elongation in thin transparent films. Each film held at a certain elongation in a jig was allowed to swell in a penetrating but nondissolving liquid. Seven liquids with different refractive indices were employed. The thickness of the film in each swelling liquid was obtained once equilibrium was established. At each elongation, from 0 to 16%, a Wiener curve was obtained. The minima of the Wiener curves yielded the intrinsic birefringence of F‐actin as a function of elongation. The intrinsic birefringence increases with elongation up to 16%, above which the thin films break. The form birefringence at a set refractive index also increases with elongation. The implication of the strain birefringence of F‐actin is discussed as it affects the optical properties, mainly light scattering, of tissues such as the fiber cells of lens of the eye.