Block copolymers of the Poloxamer type EO(x)PO(y)EO(x) (with EO = ethylene oxide and PO = propylene oxide) can form lyotropic liquid crystalline phases in water, as is known for small. hydrocarbon surfactants. The phase behavior of PE6200 (EO(6)-PO36-EO(6)) is investigated in more detail for concentrations up to 50 wt% and temperatures from 0 to 100 degrees C. The technical product contains about 10 wt% hydrophobic contaminations, which induce clouding at ambient temperatures. These "impurities" can be removed by filtration, but they do not affect the subsequent phase behavior. A lamellar phase (L(alpha)) and a spongelike phase (L(3)) are found and characterized by DSC, SANS, electric birefringence, dynamic light scattering, and specific conductivity. The temperatures for the phase transitions decrease linearly with increasing NaCl concentration. In the L(3) phase the orientational relaxation time from electric birefringence decreases with the third power of the volume fraction and the reduced diffusion coefficient derived from dynamic light scattering increases linearly with Poloxamer concentration. The conductivity of added electrolyte clearly shows the bicontinuous structure of the Poloxamer L(3) phase. Comparison of the systems PE6200/water and C(12)EO(5)/water shows analogies for the phase behavior and the properties in the different phases.