Two-dimensional crystals of dimeric photosynthetic reaction centre-LH1-PufX complexes have been analysed by cryoelectron microscopy. The 8.5A resolution projection map extends previous analyses of complexes within native membranes to reveal the alpha-helical structure of two reaction centres and 28 LH1 alphabeta subunits within the dimer. For the first time, we have achieved sufficient resolution to suggest a possible location for the PufX transmembrane helix, the orientation of the RC and the arrangement of helices within the surrounding LH1 complex. Whereas low-resolution projections have shown an apparent break in the LH1, our current map reveals a diffuse density within this region, possibly reflecting high mobility. Within this region the separation between beta14 of one monomer and beta2 of the other monomer is approximately 6A larger than the average beta-beta spacing within LH1; we propose that this is sufficient for exchange of quinol at the RC Q(B) site. We have determined the position and orientation of the RC within the dimer, which places its Q(B) site adjacent to the putative PufX, with access to the point in LH1 that appears most easily breached. PufX appears to occupy a strategic position between the mobile alphabeta14 subunit and the Q(B) site, suggesting how the structure, possibly coupled with a flexible ring, plays a role in optimizing quinone exchange during photosynthesis.