Neutron powder diffraction measurements of fully deuterated protein C-phycocyanin have been made at three temperatures, 295, 200, and 77 K, using dry and partially hydrated samples. The average coherent structure factors and the corresponding radial distribution functions d(r) are determined. The changes in d(r) functions observed in hydrated samples depend strongly on the level of hydration and most of these changes are due to water-protein interactions. At 0.365 gram D(2)O per gram of protein, the water crystallized into hexagonal ice at 200 K and below, but at 0.175 gram D(2)O per gram of protein, no crystallization of water was observed. At the higher hydration a peak appears in the radial distribution function which indicates that the average distance of the water molecule in the first hydration shell from the amino acid residues is 3.5 A.