The mechanical detection of charge has a long history, dating back more than 200 years to Coulomb's torsion-balance electrometer . The modern analogues of such instruments are semiconductor-based field-effect devices, the most sensitive of which are cryogenically cooled single-electron transistors . But although these latter devices have extremely high charge sensitivity, they suffer from limited bandwidth and must be operated at millikelvin temperatures in order to reduce thermal noise. Here we report the fabrication and characterization of a working nanometre-scale mechanical electrometer. We achieve a charge sensitivity of 0.1 e Hz sup -0.5, competitive with conventional semiconductor field-effect transistors; moreover, thermal noise analysis indicates that the nanometre-scale electrometer should ultimately reach sensitivities of the order of 10 sup -6 e Hz sup -0.5, comparable with charge-detection capabilities of cryogenic single-electron transistors. The nanometre-scale electrometer has the additional advantages of high temperature (>or= to 4.2 K) operation and response over a larger bandwidth, from which a diversity of application may result.