The development of a portable gamma ray spectrometer based on a CsI(Tl) scintillator (1.8 cm×1.8 cm×4 cm) with integralp‐i‐ndiode (1.8 cm×4 cm) is described. A single board computer containing the MC68HC11 microcontroller, a single‐chip self‐contained computer system, is used for system control. The total size of the instrument is only 12 in×7 in. including the spectrometer and power supply. The system provides a low cost, low power gamma ray spectrometer as compared to the more common PMT‐based devices. Spectra can be collected in daily intervals for up to 1 week. Special software which monitors the proper working of the spectrometer insures long term stability. This spectrometer can be used for routine monitoring and detection of gamma ray emitting radio nuclides. Performance of the spectrometer as well as gamma ray spectra are presented. The qualitative and quantitative reliability have shown its potential as a stand alone field monitoring instrument due to its low power consumption and intelligence. ©1995 American Institute of Physics.