The enzymically catalyzed conversion of chorismate to prephenate may proceed through either a chair-like or a boat-like transition state. To distinguish between these alternatives, we have prepared a series of structural analogues of the two possible transition state structures and tested them as inhibitors of chorismate mutase-prephenate dehydrogenase from Escherichia coli K12. The results indicate that the enzymically catalyzed reaction passes through a chair-like intermediate. None of the compounds studied is an ideal transition state analogue; it seems likely that the partial bond structure of the transition state precludes the corresponding orientation of the side chain in stable molecules. Nevertheless, the new inhibitors are stronger than any previously available, and the degree of inhibition is consistent with bacteriostatic activity recently observed in some of the compounds.