Euglene gracilis (strain Z) was found to contain five polyamines which could be separated by high-pressure cation-exchange chromatography. 1,3-Diaminopropane, putrescine, norspermidine (N-(3-aminopropyl)-1,3-diaminopropane), spermidine and norspermine (N,N'-bis(aminopropyl)-1,3-diaminopropane) were identified. Biosynthesis of putrescine in E. gracilis proceeds through decarboxylation of L-ornithine, no arginine decarboxylase (EC 188.8.131.52) activity could be detected. The properties of the enzymes ornithine decarboxylase (EC 184.108.40.206) and S-adenosylmethionine decarboxylase (EC 220.127.116.11) in this alga were found to be similar to those of the enzymes isolated from animal tissues or yeast cells. A bioxynthetic scheme is proposed which relates the different polyamines occurring in E. gracilis.