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In this paper a fragile watermarking technique based on Karhunen-Loève transform (KLT) and genetic algorithms (GA) is proposed. To achieve high sensibility to content manipulations, the proposed algorithm slightly modifies the middle-frequency KLT coefficients while maintaining the visual imperceptibility of the watermark (i.e., the average peak signal-to-noise ratio reported was 52.49 dB). It uses a GA to compensate the errors in the extraction of the authentication information, which is a pseudo-random generated watermark sequence. The experimental tests carried out show that the scheme is able to detect pixel-level alterations localizing the block in which the pixel(s) was modified.