The NS3 protein of hepatitis C virus contains a bipartite structure consisting of an N-terminal serine pro tease and a C-terminal DEAD bos helicase. We show that the C-terminal domain has ATPase and panhelicase activities. The integrity of the helicase function is dependent on the conserved DEAD motif and can be abolished by a His-Ala point mutation, leaving a fully functional nucleoside triphosphatase.