The isothermal quasi-ternary-phase diagram of the lecithin -cyclohexane - water system was determined at 25 degrees C using a combination of polarizing microscopy, small-angle X-ray diffraction, and NMR techniques. The system contains four lyotropic liquid-crystalline phases and two isotropic liquid phases. Apart from the lamellar (L-alpha) phase, there are only reverse-type aggregates with a water interior, in addition to an essentially pure water phase, whose relative locations in the phase diagram follow the sequence (from the oil corner to the surfactant corner): reverse micellar solution (L-2), reverse anisotropic nematic (N-2), reverse micellar cubic (I-2), reverse hexagonal (H-2), and finally, the lamellar phase. The aggregates have a finite swelling with water, and coexistence with excess water is found at higher water contents. The area per lecithin molecule was determined in the H-2 and L-alpha phases,This area varies with the mole ratio [H2O]/[Lec] = W-0 at lower W-0 values, but saturates at an area of 90 Angstrom(2)/ molecule for W-0 greater than or equal to 15. The phase diagram is discussed in relation to the known formation of giant wormlike reverse micelles in the Liquid L-2 phase. Of particular interest here is the transition from liquid (L-2) to nematic (N-2) as the wormlike aggregate concentration is increased.