The respiratory chain complexes I, III and IV (CI, CIII and CIV) are present in the bacterial membrane or the inner mitochondrial membrane and have a role of transferring electrons and establishing the proton gradient for ATP synthesis by complex V. The respiratory chain complexes can assemble into supercomplexes (SCs), but their precise arrangement is unknown. Here we report a 5.4 Å cryo-electron microscopy structure of the major 1.7 megadalton SCI1III2IV1 respirasome purified from porcine heart. The CIII dimer and CIV bind at the same side of the L-shaped CI, with their transmembrane domains essentially aligned to form a transmembrane disk. Compared to free CI, the CI in the respirasome is more compact because of interactions with CIII and CIV. The NDUFA11 and NDUFB9 supernumerary subunits of CI contribute to the oligomerization of CI and CIII. The structure of the respirasome provides information on the precise arrangements of the respiratory chain complexes in mitochondria.