Arthropod hemocyanins are large respiratory proteins that are composed of up to 48 subunits (8 × 6-mer) in the 75 kDa range. A 3D reconstruction of the 1 × 6-mer hemocyanin from the European spiny lobster Palinurus elephas has been performed from 9970 single particles using cryoelectron microscopy. An 8 Å resolution of the hemocyanin 3D reconstruction has been obtained from about 600 final class averages. Visualisation of structural elements such as α-helices has been achieved. An amino acid sequence alignment shows the high sequence identity (>80%) of the hemocyanin subunits from the European spiny lobster P. elephas and the American spiny lobster Panulirus interruptus. Comparison of the P. elephas hemocyanin electron microscopy (EM) density map with the known P. interruptus X-ray structure shows a close structural correlation, demonstrating the reliability of both methods for reconstructing proteins. By molecular modelling, we have found the putative locations for the amino acid sequence (597-605) and the C-terminal end (654-657), which are absent in the available P. interruptus X-ray data.