1. The optical retardation of single muscle fibres at rest and the optical properties of the large, early birefringence signal detectable during a twitch (Baylor & Oetliker, 1975, 1977a) were investigated. 2. The resting birefringence, B, which is the factor relating resting retardation, R, to the light path length through the fibre, L, was found to be 2.25 x 10-3 (i.e. R = 2.25 X 10-3 X L) and to be independent of wavelength ( lambda = 480-660 nm). 3. When the angle of incidence, psi, of the crossed polarizers with respect to the fibre axis was varied, the resting light intensity and large, early change in light intensity were related by the function sin2 psi-cos2psi. When the net phase shift, phi lambda, of a narrow longitudinal strip of fibre plus compensator was varied, the resting light intensity was described by the function (1-cos phi lambda), whereas the early change in light intensity followed sin phi lambda. These results make it likely that the optical mechanism underlying the early birefringence signal is a change in retardation. 4. When a narrow longitudinal strip of fibre was illuminated by monochromatic light in the range 480-690 nm, the magnitude of the signal varied approximately as expected if the retardation change is independent of wave-length. 5. The spatial characteristics of the signal were examined by moving a small slit of light across the fibre width as well as by measuring the signal collected from the entire fibre width as a function of wave-length. The results from both experiments support the idea that the large, early change in retardation is due to a volume-related rather than surface-related structure. 6. Under the assumption that the retardation change is distributed as fibre volume, its average magnitude was calculated. For fibres in normal Ringer the peak of the early retardation change compared with resting is about 1.7 x 10-3, and for fibres in D2O Ringer about 0.7 x 10-3.