<jats:p> Four aquatic plants were tested for the occurrence of unusual polyamines. The leaves of the aquatic plants tested ubiquitously contained homospermidine in addition to usual polyamines such as diaminopropane, putrescine, cadaverine, spermidine, spermine, and agmatine. Brasenia schreberi and Nuphar japonicum belonging to the family Nymphaeaceae contained aminopropylhomospermidine. Norspermidine and norspermine were detected in the blackweed Hydrilla verticillata belonging to Hydrocharitaceae. Thermospermine was detected in Brasenia schreberi. A novel tetraamine, N,N'-bis(3-aminopropyl)-1,2-ethanediamine (NH<jats:sub>2</jats:sub>(CH<jats:sub>2</jats:sub>)<jats:sub>3</jats:sub>NH(CH<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>NH(CH<jats:sub>2</jats:sub>)<jats:sub>3</jats:sub>NH<jats:sub>2</jats:sub>), was discovered in the aquatic plant Nuphar japonicum. This is the first report of the occurrence of N<jats:sup>4</jats:sup>-methylspermidine (NH<jats:sub>2</jats:sub>(CH<jats:sub>2</jats:sub>)<jats:sub>3</jats:sub>N(CH<jats:sub>3</jats:sub>)(CH<jats:sub>2</jats:sub>)<jats:sub>4</jats:sub>NH<jats:sub>2</jats:sub>) in the water chestnut Trapa natas belonging to the family Hydrocaryaceae. Key words: aquatic plants, polyamine, bis(aminopropyl)ethandiamine, methylspermidine.