A resonant, two-wavelength heterodyne interferometer has been developed to measure state densities of specific atomic populations. Using a tunable diode laser, one of the wavelengths is set near an absorption line and is sensitive to resonant enhancement of the refractive index. The other wavelength is at least several linewidths away and is used to measure nonresonant effects. The subtraction of the phase shifts of the two interferometers allows one to measure the population density in the lower state of the line being investigated. The interferometer system has been tested using a plasma, created by an inverse pinch plasma source, to measure the state density of Hn=2 by making resonant observations near the Hα(6563Å) transition. Average line-of-sight densities of 5.4×1011cm-3 were measured with a minimum resolvable density of 7.3×109cm-3. The transverse spatial resolution was 1–2mm. The technique described is applicable for measuring lower state densities in any atom or ion where the oscillator strengths are known for a given transition within the tuning range of the laser.