The rheological properties of mixtures of cationic surfactant (dialkyldimethylammonium chloride) and water are studied for a range of surfactant concentrations. In particular, we investigate rheological signatures of the conformational transformation of the system from lamellar sheets to lamellar vesicles under the action of shear. For a range of rheological test regimes, including transient, steady, and oscillatory shear, this transition is signaled by a critical stress or yield stress involving step changes in sample strain. viscosity, or dynamic moduli. However, only a portion of the sample undergoes the transition, resulting in lamellar sheets coexisting with vesicles. We further show that the transition is reversible in that ramping the stress down from above the critical stress value results in the disappearance of all of the vesicles. The final viscosity of the mixture can he higher or lower than that for the initial untreated material-possibly reflecting an imbalance in the net generation or annealing of defects in the lamellar structure by the shear treatment. Oscillatory rheometry indicates that a minimum strain is also required to obtain a phase transition. For transient shear, this corresponds to a minimum residence time in the shear field. In general, the critical stresses increase with the surfactant concentration, though they are relatively constant at 15-20 Pa for concentrations between 20 and 40 wt %.