The hepatitis B virus polymerase open reading frame, as well as various subdomains of polymerase, was expressed in insect cells using the recombinant baculovirus expression system. Full-length polymerase was expressed at very low levels in a Spodoptera frugiperda cell line, the amino-terminal domain of polymerase was expressed at high levels, and other constructs were expressed at intermediate levels. Infections of a Trichoplusia ni cell line with the same recombinant baculoviruses resulted in high levels of protein production for all polymerase constructs. Each of the polymerase polypeptides was phosphorylated in insect cells. Since polypeptides with non-overlapping sequences were phosphorylated, polymerase must be phosphorylated at a minimum of two sites.