Transposition mutations are typically associated with the activities of transposable elements such as transposons and insertion sequences, whose mobility is dependent upon transposase enzymes that catalyze exchanges between element ends and target sites. We describe a single transposition event in which a block of donor sequence is inserted at a target site without the involvement of any known transposase or the ends of any known transposable element. We propose that this is a new type of spontaneous mutation which may be difficult to detect in standard mutant hunts but may be of evolutionary importance.