Five amino acids have been isolated and newly characterized from seed of Aesculus californica. The major component of the free amino acid pool of the seed is 2-amino-4-methylhex-4-enoic acid. It is accompanied by smaller quantities of its γ-glutamyl peptide, the corresponding saturated amino acid (homoisoleucine), and its hydroxy derivative. The fifth new compound has been assigned the structure β-(methylenecyclopropyl)-β-methylalanine. The properties of each compound are described. The role of 2-amino-4-methylhex-4-enoic acid as an analog of phenylalanine has been considered in relation to the activity of the phenylalanyl-sRNA synthetase enzymes of Aesculus and other plant species.