A novel cyclic amino acid was detected in and subsequently isolated from extremely halophilic species of the bacterial genus Ectothiorhodospira. The structure of this new compound was elucidated by a combination of nuclear magnetic resonance (NMR) techniques and mass spectrometry. 1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid is only accumulated within the cytoplasm under certain growth conditions and seems to serve an osmoregulatory function. There is no previous reference to this molecule in the chemical literature and we, therefore, propose to use the trivial name 'ectoine', due to its discovery in members of the bacterial genus Ectothiorhodospira. (formula: see text).