The interaction between an anionic surfactant, sodium dodecyl sulfate (SDS), and a self-assembling ethylene oxide-propylene oxide-ethylene oxide triblock copolymer was investigated by phase diagram determination, by NMR quadrupole splittings, and by self-diffusion. Addition of SDS induces a breakdown of anisotropic liquid crystalline phases into isotropic solutions, which especially at higher concentrations are bicontinuous. At moderately high copolymer concentrations there is a transition from bicontinuous isotropic solutions to solutions of discrete micelles on addition of SDS.