The fruit of Pentadiplandra brazzeana Baillon contains a small, sweet- tasting protein named brazzein. The structure of brazzein in solution was determined by proton nuclear magnetic resonance spectroscopy at pH 5.2 and 22 °C. The brazzein fold, which contains one α-helix and three strands of antiparallel β-sheet, does not resemble that of either of the other two sweet-tasting proteins with known structures, monellin and thaumatin. Instead, the structure of brazzein resembles those of plant γ-thionins and defensins and arthropod toxins. Sequence comparisons predict that members of a newly-identified family of serine proteinase inhibitors share the brazzein fold.