The construction of a high energy resolution gamma ray detector consisting of a scintillator/mercuric iodide photodetector combination has been investigated. Several HgI/sub 2/ photodetectors have been fabricated and tested with standard NIM electronics. The energy resolution of a scintillator/HgI/sub 2/ pair was found to be 4.75%, full width at half maximum, for 662 keV /sup 137/Cs gamma ray photons. Of five detectors fabricated with our new technique, all produced resolutions better than 5.6% FWHM. This technology makes it possible to reliably produce high quality HgI/sub 2/ photodetectors. New design concepts for the HgI/sub 2/ photocell, including the transparent entrance electrode, detector geometry, and detector packaging, are described in the paper. Advantages of gamma ray spectrometers based upon crystal scintillators optically coupled to HgI/sub 2/ photodetectors (in contrast to coupling the scintillators to the more conventional light sensors, i.e., photomultiplier tubes (PMTs)) include greater ruggedness, improved energy resolution, markedly smaller size and weight, reduced power, and insensitivity to magnetic field perturbations. >