Shear thinning of ternary microemulsions of didodecyldimethylammonium bromide (DDAB), wafer, and dodecane in the range 10(3) to 10(6) s(-1) is documented over a wide range of compositions. A marked transition in the characteristic shear rate is observed accompanying the structural transition from bicontinuous to water-in-oil (w/o) droplets as previously reported from diffusion and small-angle scattering techniques. Results in the w/o region are consistent with those for a dispersion of hard spheres. The shear thinning of the bicontinuous structure, however, occurs at much higher shear rates, implying a shorter time and length scale for disruption of the equilibrium liquid structure by the flow field.