Most helicases studied to date have been characterized as oligomeric, but the relation between their structure and function has not been understood. The bacteriophage T7 gene 4 helicase/primase proteins act in T7 DNA replication. We have used electron microscopy, three-dimensional reconstruction, and protein crosslinking to demonstrate that both proteins form hexameric rings around single-stranded DNA. Each subunit has two lobes, so the hexamer appears to be two-tiered, with a small ring stacked on a large ring. The single-stranded DNA passes through the central hole of the hexamer, and the data exclude substantial wrapping of the DNA about or within the protein ring. Further, the hexamer binds DNA with a defined polarity as the smaller ring of the hexamer points toward the 5' end of the DNA. The similarity in three-dimensional structure of the T7 gene 4 proteins to that of the Escherichia coli RuvB helicase suggests that polar rings assembled around DNA may be a general feature of numerous hexameric helicases involved in DNA replication, transcription, recombination, and repair.