The enzymes of the terminal steps of phenylalanine and tyrosine biosynthesis, chorismate mutase, prephenate dehydratase, arogenate dehydratase, prephenate dehydrogenase and arogenate dehydrogenase, were studied in 11 different species of the genus Flavobacteria. A comparison of the specific activities, cofactor specificity and regulation of the enzymes, allows a differentiation within the Flavobacteria. All strains studied utilize both arogenate and p-hydroxyphenylpyruvate as an intermediate in L-tyrosine synthesis. Phenylpyruvate was found to be the precursor of phenylalanine in most bacteria. No feedback inhibition of arogenate dehydrogenase by phenylalanine and tyrosine was observed. The diverse strains of the flavobacteria were found to possess different regulatory patterns with respect to the action of phenylalanine and tyrosine on the other enzymes. On the basis of these results a tentative classification of the Flavobacteria within the two groups formed by the different DNA base ratios is proposed.