alpha,gamma-Diketo acids (DKA) were discovered from screening as selective and reversible inhibitors of hepatitis C virus NS5b RNA-dependent RNA polymerase. The diketo acid moiety proved essential for activity, while substitution on the gamma position was necessary for selectivity and potency. Optimization led to the identification of a DKA inhibitor of NS5b polymerase with IC(50) = 45 nM, one of the most potent HCV NS5b polymerase inhibitors reported.