The structure of the GaAs(1 1 1)B(root 19 X root 19)R23.4 degrees surface reconstruction has been determined using x-ray photoelectron spectroscopy and scanning tunneling microscopy. This structure is characterized by the sixfold As crown first found by Biegelsen and co-workers. In addition, we find that both As and Ga atoms occur in antisite positions in the top bilayer, and possibly in underlying layers as well. These antisite atoms are proposed to enhance the island formation involved in the epitaxial growth self-assembling nanostructures, and to interfere with the layer-by-layer growth that characterizes homo-and heteroepitaxy on most GaAs surfaces. Unlike several earlier structures proposed for this reconstruction, we propose that all of the As (Ga) dangling orbitals are filled (empty), consistent with the electron counting rule.