Electron micrographs of RecA-DNA filaments, formed under several different conditions, have been analyzed and the filament images reconstructed in three dimensions. In the presence of ATP and a non-hydrolyzable ATP analog. ATP-gamma-S, the RecA protein forms with DNA a right-handed helical complex with a pitch of approximately 95 A. The most detailed view of the filament was obtained from analysis of RecA filaments on double-stranded DNA in the presence of ATP-gamma-S. There are approximately six subunits of RecA per turn of the helix, but both this number and the pitch are variable. From the examination of single filaments and filament-filament interactions, a picture of an extremely flexible protein structure emerges. The subunits of RecA protein are seen to be arranged in such a manner that the bound DNA must be partially exposed and able to come into contact with external DNA molecules. The RecA structure determined in the presence of ATP-gamma-S appears to be the same as the "pre-synaptic" state that occurs with ATP, in which there is recognition and pairing between homologous DNA molecules.