The self-organization of n-dodecylurea poly(propylene imine) dendrimeric derivatives of the fourth and fifth generation has been investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The nonprotonated derivatives melt into isotropic liquids on heating. However, upon protonation of the tertiary amino groups in the interior of the dendritic moieties, a body-centered-cubic phase of Ia3d space group symmetry is observed. It is proposed that the polar segments have a nearly spherical conformation and are located on the axes of the gyroid surface, while the alkyl chains are in a disordered conformation extending on each side of the minimal surface.