A series of 2-(3-thienyl)-5,6-dihydroxypyrimidine-4-carboxylic acid inhibitors of the hepatitis C virus (HCV) NS5B polymerase enzyme are reported. Sulfonyl urea substituted analogs in this series proved to be the most potent active site non-nucleoside inhibitors of NS5B reported to date. These compds. had low nanomolar enzyme inhibition across HCV genotypes 1-3 and showed single digit micromolar inhibition in the HCV replicon assay. This improved cell-based activity allowed the binding mode of these compds. to be probed by selection of resistant mutations against compd. 21 (I). The results generated are in broad agreement with the previously proposed binding model for this compd. class.