Abstract Purified Bacillus subtilis DnaG primase (predicted molecular mass 68.8 kDa) behaves as a monomer in solution. We demonstrate that DnaG physically interacts with bacteriophage SPP1 hexameric helicase G 40P (G 40P 6) in the absence of ATP. G 40P 6-ATP forms an unstable complex with ssDNA, and by itself carries out ATP-driven translocation along a ssDNA template with low processivity. The presence of DnaG in the reaction mixture increased the helicase activity of G 40P 6 about 3-fold, but not the ATPase activity. The results presented here suggest that the DnaG protein stabilises the G 40P 6-ssDNA complexes.