Recently a new virus has been described which infects woodchucks, Marmota monax. This virus, named woodchuck hepatitis virus (WHV) is closely related to human hepatitis virus (HBV). The virions have the same principal antigenic system involving surface and core determinants and a serological relationship has been found. WHV has also a DNA polymerase associated with the core. It has previously been reported that trisodium phosphonoformate (PFA) but not phosphonoacetic acid (PAA) inhibits DNA polymerase associated with HBV. This investigation shows the same type of inhibition pattern by PFA and PAA on WHV DNA polymerase.