Semi-fragile watermarks are used to detect unauthorised changes to an image, whereas tolerating allowed changes such as compression. Most semi-fragile algorithms that tolerate compression assume that because compression only removes the less visually significant data from an image, tampering with any data that would normally be removed by compression cannot affect a meaningful change to the image. Scalable compression allows a single compressed image to produce a variety of reduced resolution or reduced quality images, termed subimages, to suit the different display or bandwidth requirements of each user. However, highly scaled subimages remove a substantial fraction of the data in the original image, so the assumption used by most semi-fragile algorithms breaks down, as tampering with this data allows meaningful changes to the image content. The authors propose a scalable fragile watermarking algorithm for authentication of scalable JPEG2000 compressed images. It tolerates the loss of large amounts of image data because of resolution or quality scaling, producing no false alarms. Yet, it also protects that data from tampering, detecting even minor manipulations other than scaling, and is secure against mark transfer and collage attacks. Experimental results demonstrate this for scaling down to 1/1024th the area of the original or to 1/100th the file size.