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We describe the design and performance of a dual-modality system for small animal imaging combining a high resolution SPECT component with an X-ray CT component. All components are mounted on a barrel type gantry which is able to rotate in 0.001 degrees increments about the animal. The SPECT subsystem consists of a four custom-built gamma cameras. The cameras can be equipped with either parallel hole or pinhole collimators depending on the study in question. The cameras can be operated in pairs to implement a unique half cone geometry setup in order to decrease the overall acquisition time as well as maximize spatial resolution. The average intrinsic spatial and energy resolutions of the gamma detectors is 1.9 mm and 17% respectively. The X-ray CT subsystem combines various interchangeable microfocus X-ray sources with either a high resolution CCD based detector or a flat panel Hamamatsu C7942 CMOS array. The CCD detector employs a 7k by 4k Phillips CCD chip with dimensions of 5 cm times 9 cm, large enough to require no minification between the Gd2O2S phosphor and the chip. The CT component has been installed and successfully used in a variety of small animal studies.