Drag reduction, shear and extensional rheometry, and cryogenic transmission electron microscopy (cryo-TEM) were used to study aqueous solutions of one cationic surfactant, Arquad 16-50 (5 mM), with three isomeric counterions, 2-, 3-, or 4-Cl-benzoate at 12.5 mM. Each isomer showed different; types of rheological and drag reduction behavior and different micellar structures. The 4-Cl system showed good drag reduction, high apparent extensional viscosity, and a thread-like micellar network, while the 2-Cl system showed no drag reduction, low apparent extensional viscosity, and only spherical micelles. The 3-Cl system was drag reducing and had high extensional viscosities at 30 degrees C. However, at 20 degrees C, the 3-Cl solution precipitated at high shear or extensional rates, leading to loss of drag reduction and low apparent extensional viscosity. The 3-Cl at 20 degrees C showed threadlike structures in some pictures and vesicles in others, presumably because of variations in the level of shear the samples were subjected to during sample preparation. The differences in behavior are explained by the position of the chlorine group on the benzoate ring. The hydrophobic chlorine in the 2-Cl system must reside in an unfavorable position in the aqueous phase and hence only spherical micelles are formed, which leads to no drag reduction and very low apparent extensional viscosity. The chlorine groups in the 3-Cl and 4-Cl counterions reside in the nonpolar hydrocarbon core of the micelles and, hence, stable elongated micelles can form. A schematic micellar phase diagram is proposed to summarize the transformations of surfactant molecular aggregates at different temperatures and external forces.