A combination of cDNA cloning and S1 analyses of RNA isolated from maize seedlings that carry an active Ac element has been used to define the Ac transcript. The primary transcript contains 4 introns that are excised to give a processed message which we predict to be approximately 3.4 kb. There are a number of transcription initiation sites clustered within a 90 base region about 300 bases from one end of the element. The first ATG is 600–690 bases from the transcription start and precedes an open reading frame of 807 amino acids— the putative transposase. The transcript extends to within 261 bases from the other end of the element. S1 analysis of RNa from a transgenic tobacco plant carrying an intact copy of the Ac element demonstrated a transcript identical to that in maize, although the preferred initiation sites differ.