Under application of an electric field E greater than or similar to 0.4 kV/mm, suspensions of the synthetic clay Na-fluorohectorite in a silicone oil aggregate into chain/columnlike structures parallel to E. This microstructuring results in a transition in the suspensions' rheology, from Newtonian to a shear-thinning with a significant yield stress. We study this electrorheology (ER) as a function of E and of the particle volume fraction Phi on samples with a large clay particle polydispersity. The flow curves under fixed shear rate are well fitted by the Cho-Choi-Jhon model [M. Cho et al., Polymer 46, 11484 (2005); H. J. Choi and M. Jhon, Soft Matter 5, 1562 (2009)]; proper scaling of E and of the measured shear stress provides a collapse of all flow curves onto a master curve. The corresponding dynamic yield stress scales as E(1.93), while the static yield stress inferred from disruption tests behaves as E(1.58). The bifurcation in the rheology when letting the flow evolve under constant shear stress is also characterized; the corresponding bifurcation yield stress scales as E(alpha) with alpha similar or equal to 0.5-0.6. All measured yield stresses increase with Phi; for the static yield stress, a scaling law Phi(0.54) is found. The three mutually consistent types of measurements are compared with previous measurements on laponite suspensions, and the rheologies of these two types of samples are discussed in light of existing theories of the ER effect. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3579189]