Nanocrystal superstructures of barium carbonate (BaCO(3)) which exhibit unusual morphologies are obtained by the carbon dioxide vapor diffusion technique in the presence of poly(ethylene oxide)-block-eicosa aspartate (PEO-b-Asp(20)). The highly effective bioconjugate acts as a crystal growth modifier over a broad range of concentrations. Instead of commonly observed branched needles, the morphology can be systematically varied along different twinned growth patterns towards well-defined branched dumbbells. Detailed analysis of the crystal substructures with high resolution scanning electron microscopy (HRSEM) and dynamic light scattering (DLS) discloses a potential growth mechanism of the superstructures and reveals the role of the polymeric bioconjugate therein.