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 Mammalian Genetics Research Facility. The first prototype employs a single-pixel cadmium zinc telluride detector with a pinhole collimator operating in pulse counting mode. The second version employs a phosphor screen/CCD detector operating in current mode. The major system hardware includes 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 Urn and 250 urn resolutions. The resolution of the phosphor screen detector is determined by tne modulation transfer function of the phosphor screen; images with resolutions approaching 50 (im have been acquired. The system performance with the two detectors is described and recent images are presented.