β-Methylenenorleucine was identified in the carpophores of Amanita vuginata var. fulva. Examination of a two-dimensional chromatogram prepared from an alcoholic extract of A. vaginata var. fulva revealed a new spot due to an unknown amino acid, which was difficult to separate from leucine and isoleucine. This substance initially turned yellow with ninhydrin and then violet over time, and its electrophoresis behavior at pH 5.6 indicated it was a neutral amino acid. A purified amino acid extract from A. vaginata was first fractionated on a Dowex 1×2 column, yielding the unknown substance mixed with other neutral amino acids. The neutral fraction was further separated on a Dowex 50W×4 column eluted with 1.5N HCl to obtain the amino acid as the hydrochloride, which was converted to the free amino acid via a Dowex 1×8 column and recrystallized from an ethanol-water mixture. Experimental data: melting point 171°C; molecular weight (osmometry) 144.4 (calculated for C₇H₁₃O₂N: 143.17); elemental analysis (C 58.53%, H 8.9%, N 10.05%, O 22.57%) was close to the calculated values (C 58.75%, H 9.09%, N 9.79%, O 22.38%). Optical rotations [α]ᴰ²⁰ +158° (c 0.51 in 1N HCl) and [α]ᴰ²⁰ +149° (c 0.56 in H₂O) indicated an L configuration, confirmed by a positive reaction with L-amino acid oxidase. Bromine and permanganate tests showed unsaturation, and controlled catalytic hydrogenation (Ogrz and Cooper method) confirmed a single double bond. A copper reaction indicated an α-amino group. Lemieux-von Rudloff oxidation yielded formaldehyde, identified by chromotropic acid reaction and DNPH IR spectroscopy, confirming a =CH₂ group. The hydrogenated substance was converted to 2-methylpentanenitrile using sodium hypochlorite (pH 10.8), corresponding to 2-amino-3-methylhexanoic acid. Nuclear magnetic resonance analysis (trifluoroacetic acid or 5% NaOD/D₂O) of the natural amino acid, based on chemical shifts, relative areas, and spectral features, established its structure as 2-amino-3-methylenehexanoic acid, i.e., β-methylene-L-(+)-norleucine.