The acetylcholine receptor from the electric tissue of Torpedo californica is a large, integral membrane protein containing four different types of polypeptide chains. The structure of the purified receptor in detergent solution has previously been investigated by sedimentation analysis and gel filtration. Sedimentation analysis yielded a molecular weight of 250,000 for the protein moiety of the receptor monomer-detergent complex; hydrodynamic characteristics such as the Stokes radius, however, refer to the receptor-detergent complex. In this paper we report the results of our use of low-angle neutron scattering to investigate the shape of the receptor-detergent (Triton X-100 from Rohm & Haas Co., Philadelphia, Pa.) complex and separately of its protein and detergent moieties. By adjustment of the neutron-scattering density of the solvent with D2O to match that of one or the other of the moieties, its contribution to the scattering can be nearly, if not completely, eliminated. Neutron scattering from Triton X-100 micelles established that this detergent is contrast matched in approximately 18% D2O. Scattering measurements on the receptor-detergent complex in this solvent yielded a radius of gyration of the acetylcholine receptor monomer of 46 +/- 1A. The radius of gyration and molecular volume (305,000 A3) of the receptor are inconsistent with a compact spherical shape. These parameters are consistent with, for example, a prolate cylinder of dimensions (length x diameter) approximately 150 x approximately 50 A or an oblate cylinder, approximately 25 x approximately 130 A. More complex shapes are possible and in fact seem to be required to reconcile the present results with previous electron microscopic and x-ray analyses of receptor in membrane and with considerations of the function of the receptor in controlling ion permeability. The neutron-scattering data yield, in addition, an independent determination of the molecular weight of the receptor protein (240,000 +/- 40,000), the extent of Triton X-100 binding in the complex (approximately 0.4 g/g protein), and from the extended scattering curve, an approximation to the shape of the receptor-Triton X-100 complex, namely an oblate ellipsoid of axial ratio 1:4.