Structural analysis of G-protein-coupled receptors (GPCRs) for hormones and neurotransmitters has been hindered by their low natural abundance, inherent structural flexibility, and instability in detergent solutions. Here we report a structure of the human β2 adrenoceptor (β2AR), which was crystallized in a lipid environment when bound to an inverse agonist and in complex with a Fab that binds to the third intracellular loop. Diffraction data were obtained by high-brilliance microcrystallography and the structure determined at 3.4Å/3.7Å resolution. The cytoplasmic ends of the β2AR transmembrane segments and the connecting loops are well resolved, whereas the extracellular regions of the β2AR are not seen. The β2AR structure differs from rhodopsin in having weaker interactions between the cytoplasmic ends of transmembrane (TM)3 and TM6, involving the conserved E/DRY sequences. These differences may be responsible for the relatively high basal activity and structural instability of the β2AR, and contribute to the challenges in obtaining diffraction-quality crystals of non-rhodopsin GPCRs.