Unlike any other known plant transposon, the maize transposable element Bs1 is similar to the retrotransposons previously described in yeast and Drosophila. Bs1 is bounded by 302 bp identical long terminal repeats (LTRs), and it contains open reading frames with apparent amino acid sequence similarity to reverse transcriptase and other retroviral pol gene enzymes. Bs1 is 3203 bp long, very short for a retrotransposon, and the apparent organization of its genetic information is significantly different from any previously described element. Although transcription of Bs1 has not been observed, it is probably an active transposon, since it was observed to transpose in a maize line that contains only two sequences hybridizing to Bs1 probes. Both of these sequences share 35 restriction sites with the cloned Bs1 element, and thus must be very similar or identical with it.