We are examining the behavior of the maize transposable element Ac in transgenic tomato with the goal of developing an efficient insertional mutagenesis system. Among the self progeny of a transgenic tomato plant containing an active Ac element, we identified six plants that contained the same germinally transposed Ac. In one of these plants, we found a second Ac element inserted in the same orientation and approximately 2 kb to the 5′ side of the original Ac insertion. Transmission of this composite structure was significantly reduced with less than one-quarter of the self progeny inheriting Ac either in the form of the intact parental allele (two neighboring Ac's) or derivatives of it. The derivative alleles that arose were complex in structure and could not be explained solely on the basis of the excision of one or the other Ac element. These results illustrate the potential of transposable elements to cause genetic instabilities and complex chromosomal rearrangements.