A high-resolution, far ir lamellar grating interferometer that operates in either a single-beam or a double-beam differencing mode of operation is described. The instrument covers a frequency range of 10 cm(-1) to 125 cm(-1) (1000 micro to 80 micro). To illustrate the general performance of the instrument the pure rotational spectrum of water vapor between 15 cm(-1) and 115 cm(-1) is presented. It is estimated that the absorption line centers of strong isolated lines are measured to within +/-0.008 cm(-1). To illustrate the resolution of the instrument, low wavenumber portions of the pure rotational spectrum of DCl are shown. The Cl(35)-Cl(37) isotope splitting of the J = 2 --> 3 transition (32.3 cm(-1)) is clearly resolved. The calculated separation of these two lines is 0.094 cm(-1).