Helicases that unwind DNA at the replication fork are ring-shaped oligomeric enzymes that move along one strand of a DNA duplex and catalyze the displacement of the complementary strand in a reaction that is coupled to nucleotide hydrolysis. The helicase domain of the replicative helicase-primase protein from bacteriophage T7 crystallized as a helical filament that resembles the Escherichia coli RecA protein, an ATP-dependent DNA strand exchange factor. When viewed in projection along the helical axis of the crystals, six protomers of the T7 helicase domain resemble the hexameric rings seen in electron microscopic images of the intact T7 helicase-primase. Nucleotides bind at the interface between pairs of adjacent subunits where an arginine is near the gamma-phosphate of the nucleotide in trans. The bound nucleotide stabilizes the folded conformation of a DNA-binding motif located near the center of the ring. These and other observations suggest how conformational changes are coupled to DNA unwinding activity.