De novo protein design provides an attractive means for testing and refining the principles governing the stability and tertiary structure of proteins. We describe the NMR solution structure of a 35-residue peptide designed to form a helix-loop-helix that dimerizes into a four-helix bundle. Structures were calculated on the basis of 834 NMR-derived restraints including 140 long-range NOEs. With 24 restraints per residue, the structure is well determined (0.28 Angstrom RMSD for backbone residues 3-33) and includes many features of the design yet adopts a novel topology that was unexpected. The forces that caused this peptide to adopt this unique fold are discussed.