Self-assembly in mixtures of cationic and anionic surfactants occurs synergistically because of attractive interactions between the oppositely charged headgroups. Here, such effects are exploited to obtain highly viscoelastic fluids at low total surfactant concentration. The systems considered are mixtures of the C18-tailed anionic surfactant, sodium oleate (NaOA), and cationic surfactants from the trimethylammonium bromide family (CnTAB). In particular, mixtures of NaOA and C8TAB show remarkably high viscosities: for 3% surfactant, the zero-shear viscosity η0 peaks at ca. 1800 Pa·s for a weight ratio of 70/30 NaOA/C8TAB. The high viscosities reflect the growth of giant, entangled wormlike micelles in the solutions. Mixtures of NaOA with a shorter-chain analogue (C6TAB) have much lower viscosities, indicating a weak micellar growth and hence a weak attraction between the surfactants. On the other hand, increasing the CnTAB tail length to n = 10 or 12 leads to much stronger interactions between these surfactants and NaOA. Consequently, both micellar and bilayer structures are formed in these mixtures, and the samples separate into two or more phases over a wide composition range. Thus, the synergistic growth of wormlike micelles in cationic/anionic mixtures is maximized when there is an optimal asymmetry in the surfactant tail lengths.