In this paper, a fragile watermarking method for authentication of biomedical images is proposed in the z-transform domain based on the zero locations. The z-transform is a convenient yet invaluable tool for representing, analyzing and designing discrete-time signals and system. Our watermarking method is designed by exploiting the sensitivity of the positions of z-transform zeros on the unit circle to any tampering made on the host image, such that the watermarking system can localize the portions of a watermarked image that have been tampered maliciously with high accuracy. Simulation results are presented to demonstrate the effectiveness and efficiency of the proposed scheme.