Abstract Monodisperse solutions of bovine rhodopsin monomers, devoid of lipid, associated with a linear polyoxyethylene alcohol detergent have been prepared. The composition and homogeneity of these complexes have been determined by hydrodynamic characterisation. Each rhodopsin molecule is associated with about 110 monomers of the detergent. These rhodopsin-detergent complexes have been studied by small-angle neutron scattering. Partial or total deuteration of the detergent, as well as variation of the 2H 2O/H 2O ratio in the solvent, were used to eliminate the detergent—solvent contrast at various protein—solvent contrasts. The size and shape of the detergent micelle and of the rhodopsin-detergent complexes were shown to be independent of solvent or detergent deuteration. Mixture of selectively deuterated detergent molecules allowed us to obtain an homogeneous scattering density for the detergent part of the micelles and therefore to eliminate totally its contribution to the scattering when it is contrast matched. Neutron scattering from rhodopsin alone was then measured even in highly deuterated solvents, with low incoherent background, as for a water-soluble protein. Supplementary neutron scattering measurements on rhodopsin-dodecyl dimethylamine oxide micelles confirmed essentially the results reported by Yeager (1975). Analysis of the neutron scattering data indicates that most of the hydrophobic residues of rhodopsin form a compact region which has zero hydration, this probably being the part which is embedded in the disc membrane, and that the unhydrated rhodopsin molecule is asymmetrically arranged with respect to the membrane. Comparison with the results of a small-angle X-ray scattering study (Sardet et al., 1976) implies that the peripheral regions on both sides of the membrane are highly hydrated. Several schematic models are discussed.