Mixtures of the nonionic surfactant hexaethylene glycol n-hexadecyl ether, C16EO6, and water were examined by optical microscopy, H-2 NMR, and small-angle X-ray and neutron scattering. The phase diagram has been delineated between 28% and 70% by weight of surfactant. Besides the conventional lamellar, L(alpha), and hexagonal, H-1, phases, the system exhibits a wide range of "intermediate’ behavior including a defected lamellar phase (L(alpha)H), a nematic phase of rod micelles, (N(C)), and cubic or intermediate phases depending upon sample history, which both occur in the same region of the phase diagram. It is the first time that an N(C) phase has been identified in a nonionic surfactant system. The rich diversity of phases is explained in terms of the decreasing hydration of the polyoxyethylene polar head groups with increasing temperature and the increasing interaggregate interactions with increasing surfactant concentration.