We examined the enzymology and regulatory patterns of the aromatic amino acid pathway in 48 strains of cyanobacteria including representatives from each of the five major grouping. Extensive diversity was found in allosteric inhibition patterns of 3-deoxy-D-arabinoheptulosonate 7-phosphate synthase, not only between the major groupings but also within several of the generic groupings. Unimetabolite inhibition by phenylalanine occurred in approximately half of the strains examined; in the other strains unimetabolite inhibition by tyrosine and cumulative, concerted, and additive patterns were found. The additive patterns suggest the presence of regulatory isozymes. Even though both arogenate and prephenate dehydrogenase activities were found in some strains, it seems clear that the arogenate pathway to tyrosine is a common trait that has been highly conserved among cyanobacteria. No arogenate dehydratase activities were found. In general, prephenate dehydratase activities were activated by tyrosine and inhibited by phenylalanine. Chorismate mutase, arogenate dehydrogenase, and shikimate dehydrogenase were nearly always unregulated. Most strains preferred NADP as the cofactor for the dehydrogenase activities. The diversity in the allosteric inhibition patterns for 3-deoxy-D-arabinoheptulosonate 7-phosphate synthase, cofactor specificities, and the presence or absence of prephenate dehydrogenase activity allowed the separation of subgroupings within several of the form genera, namely, Synechococcus, Synechocystis, Anabaena, Nostoc, and Calothrix.