An approximately 1.4 kb fragment of DNA called Mu1, mutationally inserted into the Adh1 locus of maize in a Robertson's Mutator plant, has been cloned. The instability of the mutation induced by this element, the nature of the Robertson's Mutator system, and terminal inverted repeats indicate that the 1.4 kb insert is a transposable element. All Robertson's Mutator corn lines have Mu1-like elements, at copy numbers of 10-70 per diploid genome. The basic size of these multiple interspersed copies is generally the same. The elements are found on different genomic restriction fragments in closely related individuals, indicating a high degree of mobility. Aside from the one corn line identified by Robertson in the mid-1970s as Mutator, all maize lines tested, plus several near and remote corn relatives, have no detectable DNA which cross-hybridizes strongly with Mu1.