We have determined the nucleotide sequence of the transposable maize controlling element Activator (Ac). The Ac element is 4563 bp long and has an imperfect terminal repetition of 11 bp. The element contains two open reading frames (ORF) encoding polypeptides of 839 and 210 amino acids. Evidence derived from structural analysis of a closely related, but transposition-defective Dissociation (Ds) element indicates that the large ORF is the structural gene for a trans-acting function required for transposition. The two ORFs diverge from a short intergenic region which contains characteristic eucaryotic transcription initiation sequences.