We present an image reconstruction technique for ultrasound-modulated optical tomography. It is the first time, to the authors' knowledge, that a reconstruction technique is developed for such tomography. In analogy to X-ray computed tomography, an ultrasonic beam is scanned linearly and angularly across a biological-tissue sample. Ultrasound-modulated optical signals, reflecting the optical properties of the sample inside the ultrasonic column, are detected and taken as the projection data for the reconstruction, where a filtered back-projection algorithm is implemented. With the technique, two-dimensional images of biological tissues in cross-sections containing the scanned ultrasonic axis are obtained. The image resolution is determined by the diameter of the ultrasonic focal zone. The technique can be implemented with any standard signal-detection scheme for ultrasonic modulation of coherent light in scattering media and can be applied directly to achieve three-dimensional images of biological tissues.