Several recent studies have attempted to determine the flow stresses during geological deformations from the microstructures of rocks, utilizing quantitative estimates of such structural parameters as grain size, subgrain size, and dislocation density. The basis for these stress determinations is the apparent correlation between flow stress and microstructures in experimental deformation studies of several crystalline line materials. Estimates of flow stresses are presented for a group of quartz-bearing mylonitic rocks from the Coyote Mountain mylonite zone, near Borrego Springs, California, using the grain sizes and dislocation densities of quartz grains. Neglecting complexities in the geological history of the rocks, flow stresses inferred from grain sizes range from 16 to 74 MPa (160 to 740 bars), whereas those derived from dislocation densities range from 131 to 50 Mpa, respectively, for the same rocks. Stress estimates by the two methods are not well correlated and may differ by an order of magnitude of the same rock. The major causes of the inconsistencies are considered to be (1) inadequacy of the currently available experimental data and (2) complexity of the thermomechanical history of the rocks. These problems are discussed, and the potential value of the methods is assessed. It appears likely that the rocks were subjected to flow stresses (&sgr;1-&sgr;3) of 100 MPa (1kbar) or more during mylonitization.