An "inverse micellar"- or "water-in-oil"-like (similar to the lyotropic type I-II) thermotropic body-centered cubic (BCC) liquid crystal (LC) phase was discovered in a poly(ethyleneimine) with a degree of polymerization (DP) = 20 containing 3,3,5-tri(dodecyloxy)benzoyl minidendritic side groups. Fiber X-ray diffraction (XRD) experiments show that the extinction symbol of this phase is I--- and, alongside other evidence, suggested that the space group is Im (3) over bar m (Q(229)). These results point to a supramolecular inverse micellar-like structure with the polymer backbone and the benzoyl groups aggregated into globules located at the corners and the center of the unit cell. The n-alkyl groups radiate out of the aromatic part of the globule: and make up the continuos matrix of the lattice. The cubic unit cell parameter is a = 42.6 Angstrom. Upon shearing, the BCC phase aligns along the [111] direction. The BCC lyotropic Im (3) over bar m inverse micellar phase of I-II type has not yet been observed and the experiments reported here will access its design. The present is only the second "inverse micellar" thermotropic LC phase known. The first one was previously reported from our laboratories (Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am. Chem. Soc, 1997, 119, 1539) and has a Pm (3) over bar n symmetry. Both the Im (3) over bar m and the Pm (3) over bar n cubic lattices were self-organized from spherical supramolecular dendrimers. The BCC cubic LC lattice reported hen enriches the synthetic capabilities of dendritic building blocks.