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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 (C_nTAB). 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 …