A phase diagram of the polyoxyethylene trisiloxane surfactant-water system was constructed as a function of polyoxyethylene (EO) chain length at 25 degrees C. The HLB (hydrophile-lipophile balance) of surfactant corresponds to the volume ratio of the EO chain to that of surfactant molecule (phi(EO)/phi(S)), where phi(S) and phi(EO) indicate the volume fractions of surfactant and hydrophilic moiety in the system, respectively. Aqueous micellar (Wm), hexagonal liquid crystalline (H-1), lamellar liquid crystalline (L alpha), and reverse micellar (Om) phases are formed with decreasing phi(EO)/phi(S). A sponge phase (D-2) is also formed near the L alpha phase region. The effective cross sectional area per one surfactant molecule, as, in liquid crystals in the present systems depends only on their EO chain lengths and are the same as that in ordinary linear hydrocarbon surfactant systems. Since the maximum length of the trisiloxane moiety in its extended form is short, the L alpha phase is formed at phi(EO)/phi(S) between 0.45 and 0.6 in the present system, whereas an ordinary linear-type nonionic surfactant forms the H-1 phase in the same range of phi(EO)/phi(S). The H-1 phase, which was observed in a narrow range of phi(EO)/phi(S), becomes stable upon addition of oil. Although the H-1 phase in the absence of oil is considered to have the hexagonal structure confirmed by polarized optical microscopy, the calculated radius of the cylinder is much longer than the hydrophobic chain length in its extended form. Perhaps, the present H-1 phase is different from an ordinary hexagonal liquid crystalline structure. The three-phase behaviors of microemulsions in the present systems were also examined.