A single-electron transistor has been fabricated by an optical fabrication method. A small dot, a source and drain reservoir, and in-plane gates are all built from the two-dimensional electron gas of an n-type GaAs/AlGaAs heterostructure. Laser-written p-doped lines are used to define this dot with a diameter of about 70 nm and to insulate it from the in-plane gates. Tunnel junctions connect the dot with source and drain. The in-plane gates are used to tune the tunnel junctions and to change the electrostatic potential of the dot. A large charging energy of 5 meV and clear Coulomb-blockade oscillations are observed at helium temperature, due to a small dot capacitance of about 3 × 10–17 F.