When light is diffracted by a single frog muscle fiber the intensities I kappa of the different orders kappa (kappa = 1,2,3) strongly depend on the angle between the axis of the incident beam and the fiber axis. Maximum intensity is not obtained with perpendicular incidence (omega = 0 degree) but at angles that can be calculated for each order number and sarcomere length using Bragg's formula. In analogy to techniques developed for x-ray structure analysis of mosaic crystals we have rotated the fiber around an axis perpendicular to the fiber axis and to the incident beam axis within an angular range delta omega = +/- 35 degrees and recorded the light intensities I kappa. Diffraction efficiencies defined as E kappa = integral of I kappa d omega were studied as a function of sarcomere length and during isometric contraction. The sarcomere length dependences of the efficiencies E kappa of the first three orders show characteristic trends. E1 increases with fiber stretch, E2 has a minimum at a sarcomere length near 2.8 micrometers, and E3 has a maximum near 2.5 micrometers. These trends as well as the observed efficiency ratios are in fairly good agreement with predictions by the intensity formula developed for x-ray structure analysis. During isometric contraction, the diffraction efficiencies of the fiber decrease, with the decreases becoming greater the higher the order number. These decreases might be caused by a longitudinal displacement of myofibrils of up to 0.4 micrometers. The efficiency of light diffraction strongly depends on the tonicity of the bathing fluid. Hypertonic (3/2 x normal) solution reduces E1 to less than half, hypotonic (2/3 x normal) solution increases E1 to almost twice the value obtained in normal Ringer's solution.