β-Pyrazol-I-ylalanine, an isomer of histidine, occurs in large amounts in several cucurbitaceous species. Enzymic synthesis of the new amino-acid is shown to occur by the condensation of pyrazole and serine in an analogous manner to that in which tryptophan is synthesized from indole and serine. The properties and distribution of the new enzyme, called β-pyrazol-I-ylalanine synthetase, have been studied using crude extracts of cucumber seedlings. The enzyme has also been demonstrated in extracts of other cucurbit seedlings. A chemical synthesis of β-pyrazol-I-ylalanine from serine and pyrazole adapted from the enzymic pathway has been used to demonstrate indirectly the presence of pyrazole in cucumber and melon seeds.