The atomic structures of argon-ion bombarded GaAs (001) surfaces were investigated using in situ scanning tunneling microscopy (STM). The experiments were conducted in an ultrahigh vacuum multichamber system equipped with a STM, a molecular beam epitaxy facility, as well as an argon-ion gun. MBE-grown GaAs surfaces were bombarded by argon ions of 500 eV at doses of approximately 10(13) ions cm-2, and the resulting surface structures were examined by a STM. In the STM images, most of the defects produced by the argon-ion bombardment were about 1.6 nm wide in the [110] direction. This result seems to suggest that the interaction of surfaces with argon ions during the bombardment took place in a (2X4) unit cell; that is, all of the three arsenic dimers of a (2X4) unit cell in the bombarded regions were taken away as if they comprised a single unit of interaction. The average defect density increased with increasing argon-ion dose.