An imaging technology, thermoacoustic tomograpy (TAT), was applied to the visualization of high-intensity focused ultrasound (HIFU)-induced lesions. A single, spherically focused ultrasonic transducer, operating at a central frequency of approximately 4 MHz, was used to generate a HIFU field in fresh porcine muscle. Microwave pulses from a 3-GHz microwave generator were then employed to generate thermoacoustic sources in this tissue sample. The thermoacoustic signals were detected by an unfocused ultrasonic transducer that was scanned around the sample. To emphasize the boundaries between the lesion and its surrounding tissue, a local-tomography-type reconstruction method was applied to reconstruct the TAT images of the lesions. Good contrast was obtained between the lesion and the tissue surrounding it. Gross pathologic photographs of the tissue samples confirmed the TAT images.This work demonstrates that TAT may potentially be used to image HIFU-induced lesions in biological tissues.