Neutron scattering is particulary useful when parts of a structure can be deuterated. From Halobacterium halobium we have obtained, by biosynthetic incorporation, purple membranes in which all of the valines or all of the phenylalanines are present in deuterated form. Difference Fourier techniques permit a general assessment of the distribution of valine and phenylalanine in projections of the purple membrane structure. These show that valine is distributed toward the periphery of a single bacteriorhodopsin molecule, whereas phenylalanine is distributed toward its center. We use the facts that the amino acid sequence is known and that much of it can be assigned to the alpha helices of the bacteriorhodopsin structure to interpret our results. Comparison of our maps with the distribution of valine and phenylalanine around alpha-helical perimetrs establishes the distribution of other amino acids and leads to the conclusion that the charged and polar groups of the bacteriorhodopsin molecule tend to lie at the molecular interior, away from contact with lipid, while the nonpolar surfaces are directed outward, making contact with the lipid regions. Thus, the protein is "inside-out" compared with the organization of soluble proteins.