The RecA protein of Escherichia coli is involved in recombination (for review see ref. 1). The protein binds transiently to double-stranded DNA in the presence of ATP. In the presence of ATP gamma S, a non-hydrolysable analogue of ATP, recA-DNA complexes are stable. Duplex DNA in these complexes is stretched by a factor 1.5 (ref. 4), and the complexes appear in the electron microscope as helical filaments with a pitch of approximately 100 A and 6.2 recA units per turn covering 18.6 base pairs (bp). RecA crystals have a space group of similar helical parameters. In order to understand the function of recA, it is necessary to describe the conformation of the DNA in the recA complex. Using a topological method, the present work determines the helicity of DNA in the complex. We find that the DNA helix follows the protein helix visible in the electron microscope and has 18.6 bp per turn, which corresponds to an unwinding of the DNA double helix by 15 degrees per bp.