Two versions of a new high-resolution x-ray computed tomography system are being developed to screen mutagenized mice in the Oak Ridge National Laboratory Mouse Genetics Research Facility. The first prototype employs a single pixel solid-state detector with a pinhole collimator operating in pulse counting mode. The second version employs a phosphor screen/CCD detector operating in current mode. The gantry consists of a low-energy x-ray tube, two linear translation stages and a rotational stage. For the single pixel detector image resolution is determined by the step size of the detector stage; preliminary images have been acquired at 100 μm and 250 μm resolutions. The resolution of the phosphor screen detector is determined by the modulation transfer function of the phosphor screen; images with resolutions approaching 50 μm have been acquired. The system performance with the two detectors is described and recent images are presented.