A new technique providing real-time high-speed measurements of sarcomere length from on-line analysis of the striation image has been developed. This method of measurement is not susceptible to the problems of interpretation encountered in laser diffraction. Sarcomere shortening patterns were obtained, using this method, from single toe fibres of Rana pipiens, and stepwise phenomena similar to those previously reported from laser diffraction were observed. The distribution of step size showed several peaks, the most prominent corresponding to 5.7 nm per half sarcomere.