The structure of crystalline purple membrane reconstituted from purified bacteriorhodopsin (BR) chymotryptic fragments has been studied by neutron diffraction. In one of the samples studied, the fragment C-2, encompassing the first two predicted transmembrane segments, was prepared from deuterated purple membrane. The diffraction changes when the natural C-2 fragment is substituted by a deuterated one are analysed in terms of a seven-helix model for BR. The assignment of the labelled fragment to one end of the molecule placed new constraints on folding models for the protein.