L-γ-methyleneglutamic acid (structure I) is rather common in higher plants, and its homologous L-γ-ethylideneglutamic acid was recently reported as a constituent of Tulipa gesneriana. We isolated these two amino acids from seeds of Tetrapleura tetraptera (Schum. et Thonn.) Taub. (Mimosaceae) using traditional methods including ion-exchange chromatography, assigned the structure II (a stereoisomer of the structure proposed for the tulip isolate) to our ethylidene derivative, and confirmed the structure via nuclear magnetic resonance, ultraviolet spectroscopy, optical rotatory dispersion, and pKs measurements, which were consistent with data for stereoisomeric α,β-disubstituted α,β-unsaturated carboxylic acids and supported the proposed configuration around the double bond.