A systematic analysis of the evolution of aromatic amino acid biosynthesis in the Proteobacteria, previously focussed mainly upon the γ subdivision, has now been extended to the β subdivision. Five lineages were studied, represented by Neisseria gonorrhoeae, Nitrosomonas europaea, Alcaligenes faecalis, rRNA Group-III pseudomonads/Rubrivivax gelatinosus, and rRNA Group-II pseudomonads/Rhodocyclus tenuis. Within the phenylalanine pathway, the bifunctional P-protein (chorismate mutase/prephenate dehydratase) was present in each lineage and must have evolved in a common ancestor of the β and γ subdivisions. Each P-protein was found to be subject to activation by L-tyrosine, and to feedback inhibition by L-phenylalanine. Phenylalanine-inhibited (DS-phe) and tyrosine-inhibited (DS-tyr) isoenzymes of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase probably existed in the common β-subdivision ancestor, with DS-tyr being lost in N. gonorrhoeae and A. faecalis. The participation of DS-phe in a dissociable multienzyme complex with one or more other common-pathway enzymes is known to exist in N. gonorrhoeae. The same complex is indicated by two peaks of DS-phe seen in chromatographic profiles of Group-III pseudomonads and A. faecalis. It is concluded that the contemporary DS-phe species present in subdivisions γ and β must have had independent origins. Tyrosine biosynthesis was found to be quite diverse within the β subdivision. Nit. europaea possessed an arogenate dehydrogenase which was specific for NADP+. In all other lineages, a broad-specificity cyclohexadienyl dehydrogenase (CDH) was present. In N. gonorrhoeae the CDH was specific for NAD+ while the remaining CDH species could utilize either NAD+ or NADP+. Only the CDH species within the rRNA Group-II pseudomonad/R. tenuis lineage was feedback-inhibited by L-tyrosine, and this correlated with an allosteric pattern where activation of the prephenate dehydratase component of the P-protein by L-tyrosine was relatively poor. However, the CDH enzyme present in N. gonorrhoeae and A. faecalis was subject to inhibition by 4-hydroxyphenylpyruvate, this being competitive with respect to the cyclohexadienyl substrate. The monofunctional species of chorismate mutase (CM-F) and cyclohexadienyl dehydratase, widely distributed among the γ-subdivision assemblage and recently shown to be periplasmic enzymes, were demonstrated in Pseudomonas pickettii, a member of rRNA homology Group-II.