A torsion balance, designed to measure the force on a flat plate (or other simple geometry) in a low density gas flow, is described. The balance has been used with two interchangeable torsion member sizes, with diameters of 0.00064″ and 0.00090″, which measured forces up to a maximum of 0.6 milligram and 2.4 milligrams, respectively. Calibrations (obtained in still air using weights) were reproducible within ±0.005 mg and ±0.04 mg for the two sizes. Data obtained for a flat plate in a gas stream under large molecular mean free path conditions are presented.