AbstractAliphatic linear polyamines, from diamines to hexaamines, tertiary branched tetraamines, and quaternary branched pentaamines are widely distributed in eubacteria, archaebacteria, and cyanobacteria. Twenty-four linear types and four branched types are acid extractable from bacterial cells and can be chromatographically analyzed and identified. The varieties of polyamines are due to the combination of amino acid decarboxylase activities to form diamines, aminopropyl- and aminobuty1-transfer activities mediated by aminopropy1transferases or Schiff-base complex formation, and hydroxylation activity. The absence or presence of spermidine, nor-spermidine or homospermidine and the occurrence of 2-hydroxyputrescine and diaminopropane are related to grouping into theα,β,γandδsubclasses withinProteobacteria. Flavobacteriumcomplex and green bacteria contain putrescine and homospermidine. Gram-negative thermophiles contain long linear and branched polyamines; however, their distribution profiles are species specific. Gram-positive eubacteria, which compriseBacilluscluster, anaerobes, and actinomycetes, ubiquitously contain putrescine and spermidine, while the occurrence of spermine is limited to thermophiles. Archaebacteria are separated into polyamine-absent meth-anogens and halophiles, homospermidine-dominant methanogens, spermidine-dominant methanogens, and spermidine- and norspermidine-containing thermophiles. Cyanobacteria comprise two types; one group contains homospermidine and the other spermidine. The polyamine distribution pattern can serve as a chemotaxonomic marker in bacterial classification and is associated with bacterial systematics on the level of order, family, or genus.