1. When a single muscle fibre was externally stimulated to give a propagated action potential, a large decrease in light intensity was measured with the fibre positioned between crossed-polarizers oriented at +/- 45 degrees with respect to the fibre axis. This large optical signal begins just after stimulation and its man phase precedes the development of positive tension. 2. The peak (or plateau) amplitude of the signal, expressed as the peak (plateau) change in light intensity, delta I, divided by the resting light intensity, I, was typically (minus) 1 to 3x10-3 and the time-to-peak (plateau) was 4 to 6 ms (20 degrees C). 3. When mechanical activity was minimized by stretch or Ringer replacement with D2O or hypertonic solution, the peak tension response was reduced in far greater proportion than the peak optical response, suggesting that the early optical signal is not due to changes in tension or gross fibre movement. 4. The magnitude of the optical response was increased by nitrate and double-shock stimulatin, procedures which potentiate the twitch response. 5. The optical signal was shown to propagate along the fibre length with a conduction velocity appropriate to the surface action potential. 6. The above results suggest that the large, early birefringence signal reflects a step or steps in the sequence of events leading to contractile activation.