A large number of nucleotide diphosphate sugar pyrophosphorylases have been described (for review see References 1 and 2), but only a few of these have been purified to any significant extent. In this paper we report the further purification of deoxythymidine diphosphate n-glucose pyrophosphorylase from Pseudomonas ueruginosa (3). With this partially purified enzyme, it is possible to show that this enzyme will utilize not only thymidine nucleotides as substrates but also uridine nucleotides and deoxyuridine nucleotides. The control of the enzymatic reactions involved in the biosynthesis of structural polysaccharides has only recently become a subject of investigation. In particular, Kornfeld et al. (4) have shown that UDP-GlcNAc is a feedback inhibitor of the n-fructose 6-phosphate-L-glutamine amidotransferase, and also that CMPN-acetylneuraminic acid is an inhibitor of the enzyme that cleaves UDP-GlcNAc to N-acetyl-n-mannosamine. In each case the end product inhibits the first enzyme that is unique to its biosynthetic pathway. It is shown in this paper that the dTDP-n-glucose pyrophosphorylase is subject to feedback inhibition by dTDP-Lrhamnose. In an accompanying paper, Bernstein and Robbins (5) describe related observations obtained independently on the dTDP-nglucose pyrophosphorylase and the UDP-n-glucose pyrophosphorylase from Salmonella.