Abstract Diaminopimelic acid (DAP) occurs in a combined form in the cell-wall mucopeptides of bacteria and blue-green algae and in a soluble form used as an intermediate in the synthesis of lysine. The amount of the free form is very small compared to that of the bound form. The four species of blue-green algae and four species of flexibacteria used in this investigation all contained bound DAP, in amounts ranging from 0.32% to 0.04% of the dry weight. A single determination on Beggiatoa alba indicated a DAP content of 0.08%. DAP could not be demonstrated in Chlorella pyrenoidosa, Glaucocystis nostochinearum, or Cyanidium caldarium (significance levels for these determinations were 0.0012%, 0.009%, and 0.0047% of the dry weight, respectively). Experiments with C14-labelled DAP indicated that lysine synthesis in flexibacteria occurs by decarboxylation of DAP, as in other bacteria and blue-green algae. The significance of DAP in the phylogeny of these microorganisms is discussed.