We describe a single photon emission computed tomograph (SPECT) which can be operated inside state-of-theart magnetic resonance imaging (MRI) systems. The combined SPECT/MRI system allows one to acquire simultaneously the data from both modalities and co-register the images in space and time. Unlike conventional SPECT systems, which use scintillators and photomultiplier tubes, the new SPECT is based on the semiconductor cadmium zinc telluride (CZT) and application specific integrated circuits (ASICs) - two technologies which are MR-compatible and almost insensitive to magnetic fields. The γ-radiation imaging system has a 3-cm diameter fieldof- view and allows one to acquire regional tomographic images in mice without moving the camera. We have built the SPECT prototype for small animals and pre-clinical applications. The experiments show, firstly, that the MRI is minimally affected by the SPECT inside the bore and, secondly, that the SPECT works well inside the MRI. Figures-of-merit for CZT-based pixellated radiation sensors are individual photon counting and high resolution energy spectroscopy. We measure an energy resolution of 5.4 keV full-width-half-maximum at the 140-keV photo peak from 99mTc for both inside and outside of the MRI. The new SPECT has a multi-pinhole radiation collimator with apertures of 2-mm diameter. The line spread function has 2.5-mm FWHM and 2 capillary tubes 4-mm apart can be resolved in the reconstructed image.