The resonant tunneling of electrons through a 20 nm scale InAs quantum dot bound by a pair of very thin AlAs barriers is studied. A well-resolved composite peak resulting from the ground1sstates was observed at 4.2 K in current–voltage characteristics. By investigating the effects of inplane magnetic fields, the shape of the wave function and the spatial extent of the first two electronic states are clarified. ©1997 American Institute of Physics.