Seven ellagitannins isolated from Phyllanthus myrtifolius and P. urinaria (Euphorbiaceae) have been shown, for the first time, to be active against Epstein-Barr virus DNA polymerase (EBV-DP) at the microM level. All these compounds have the same moiety of a corilagin, and differ from each other by different substitutions at C-2 and C-4 of the glucose core. SAR analysis and molecular modeling reveal that the essential pharmacophore of these tannins resides in the corilagin moiety. The outer complex carboxylic acid moieties appear to act only as auxopharmacore.