Protocatechuate is a universal growth substrate for members of the family Rhizobiaceae, and these bacteria utilize the aromatic compound via the beta-ketoadipate pathway. This report describes transcriptional controls exercised by different subgroups of the Rhizobiaceae over five enzymes that catalyze consecutive reactions in the pathway: protocatechuate oxygenase (EC 22.214.171.124), beta-carboxy-cis,cis-muconate lactonizing enzyme (EC 126.96.36.199), gamma-carboxymuconolactone decarboxylase (EC 188.8.131.52), beta-ketoadipate enol-lactone hydrolase (EC 184.108.40.206), and beta-ketoadipate succinyl-coenzyme A transferase (EC 220.127.116.11). All five enzymes were inducible in the fast-growing strains Agrobacterium rhizogenes, Agrobacterium tumefaciens, Rhizobium fredii, Rhizobium meliloti, Rhizobium leguminosarum, and Rhizobium trifolii. Specific activities in induced cells ranged from 5- to 100-fold greater than those found in uninduced cells. In contrast to the fast-growing strains and members of every other microbial taxon examined to date, the slow-growing Bradyrhizobium japonicum and cowpea Bradyrhizobium spp. constitutively expressed four of the five enzymes; protocatechuate oxygenase was the only inducible enzyme in this group. The slow-growing strains included different DNA homology groups, so it appears likely that constitutive expression of the four enzymes is a common trait in the bradyrhizobia. This property points to the importance of aromatic compounds and aromatic catabolites in the nutrition of these organisms.