The structure of α-crustacyanin, the blue carotenoprotein of lobster (Homarus gammarus) carapace, has been investigated for the first time using small-angle X-ray scattering. In this paper, we have determined the dimensions of this protein composed of eight heterodimeric subunits of β-crustacyanin. Analysis of the scattering spectra and estimation of the shape of α-crustacyanin show that the protein fits into a cylinder with an axial length of 238 Å and a radius of 47.5 Å, in which the eight β-crustacyanin molecules are probably arranged in a helical manner.