In a previous communication (Rann & Cain, 1969) we described some aspects of the regulation of the 3-oxoadipate pathway in Nocardia opaca. This organism, which has a versatile nutritional spectrum, can metabolize a wide range of aromatic precursors through the protocatechuate and catechol branches of this pathway. The inducer for the protocatechuate-branch enzymes (protocatechuate oxygenase, 3-carboxymuconate sycloisomerase, 4-carboxymuconolactone decarboxylase and 3-oxoadipate enol lactone hydrolase), all of which are co-ordinately induced, was 3-oxoadipate. Similarly, there was a co-ordinate pattern of induction in the catechol branch of the pathway between muconolactone isomerase and enol lactone hydrolase. Catechol 1 ,Zoxygenase and cis,cis-muconate cycloisomerase, although both induced by cis,cis-muconate, were independently regulated. The apparent anomaly of enol lactone hydrolase being a member of two distinct, co-ordinately induced blocks was resolved when it was observed that N. opaca formed two isoenzymes, one characteristically associated with the catechol branch and the other with the protocatechuate branch. The phenotypic result of this regulation pattern is that growth of the bacterium upon a catechol precursor such as benzoate results in the appearance of all four enzymes of the protocatechuate branch although under these circumstances they are entirely without any physiological function. These regulation patterns have subsequently been confirmed by using inducer analogues such as adipate and 3-methylglutarate and by examination of the enzyme contents of mutant strains of N. opaca blocked in each step of both pathways. In one particular mutant which failed to grow on both benzoate or protocatechuate, more than one enzyme activity was affected. Protocatechuate oxygenase was present at only 16%, 3-carboxymuconate cycloisomerase at only 0.26 %, 4carboxymuconolactone decarboxylase at 65 % and 3-oxoadipyl-CoA transferase at 93 ”/, of the amounts found in a revertant to wild-phenotype of this mutant. No serious genetic analyses have ever been carried out in this genus but these results are consistent with this mutant carrying a polarity mutation in an operon controlling the co-ordinate synthesis of the enzymes of the protocatechuate branch. On this assumption, the order of genes in this operon would be: operator; cycloisomerase; oxygenase; decarboxylase. The transferase, which does not form part of the co-ordinate block, was independently regulated (D. L. Rann, unpublished work), so would be expected to exhibit no effect of the putative polarity mutation and show the virtually unchanged enzyme content that was observed. An identical regulatory system was found in Nocardiu globerulu N.C.T.B. 9158, in Nocardia cauiue A.T.T.C. 14629 and in N. cauiae strain R1315 (Tsukamura, 1969), in Nocurdia erythropolis, in Nocardia corallina and in Mycobacterium rhodocrous strains N.C.I.B. 9259 and N55, which Goodfellow (1971) has classified as Nocardiu rzibra on the basis of an extensive numerical taxonomy study. These species share many phenotypic traits and are quite similar in their DNA base composition so that this particular control mechanism for aromatic catabolism may have had a single evolutionary origin within this group of organisms (Stanier, 1968). One nocardioform organism of uncertain taxonomic position, Jenseniu canicvuria (Bissett & Moore, 1950), has recently been classified as a mating type of N. erythropolis by Adams et al. (1970) and within N. rubra by Goodfellow (1971). It showed some