Recently, a chemical investigation of seeds of Atelia herbertsmithii led to the isolation of two achiral l-aminocyclobutane-l-carboxylic acids: 2,4-methanoglutamic acid (2) and 2,4-methanoproline (3). This paper describes the isolation and characterisation of a third achiral cyclobutane amino acid, cis-l-amino-3-hydroxymethyl-cyclobutane-l-carboxylic acid (I), from Atelia herbert-smithii Pittier (Leguminosae). Its structure was determined by spectroscopic and X-ray crystallographic methods. By a study of the H NMR spectrum of the crude extract, the relative amount of (I) to that of methanoproline in the plant was shown to be 1 to 1.15. The 3-substituents in (I), (2) and (3) are all trans to the l-carboxyl group, indicating that the compounds may be metabolically related. The seeds of four other Atelia species and of both species of the closely related genus Cathostegia contained levels of (I), (2) and (3) estimated from ionophoresis papers to be similar to those found in seeds of A. herbert-smithii; the three cyclobutane amino acids were also major components of the free amino acids in the leaves of these species.