The first example of polytypism in the I II-V semiconductors has been demonstrated with the synthesis of cubic LiZnSb by a low-temperature solution phase method. This phase exhibits a unique coloring pattern that is novel for this class of compounds. The choice of site configuration has a considerable impact on the band structure of these materials, which in turn affects the transport properties. While the hexagonal polytype has been suggested as a promising n-type and extremely poor p-type thermoelectric material, the cubic analogue is calculated to have high efficiencies for both the n- and p-type derivatives (1.64 and 1.43, respectively, at 600 K). Furthermore, the cubic phase is found to be the energetically favored polytype. This surprising result provides a rationale for the lack of success in synthesizing the hexagonal polytype in either stoichiometric or n-type compositions.