We have sequenced HFL1 from D. melanogaster, the only cloned hobo element shown to have transposase activity. The 2959 bp HFL1 sequence predicts a 2.0 kb open reading frame (ORF1) with substantial amino acid similarity to the transposases of Activator (Ac) from maize (Zea mays) and Tam3 from snapdragon (Antirrhinum majus). Mutational analysis of a C-terminal region of ORF1 conserved with Ac and Tam3 indicates that it is essential for hobo transposase activity. This is an example of extensive amino acid sequence identity between short inverted repeat elements in different kingdoms. We discuss the possibility that the conservation of hobo, Ac, and Tam3 transposases represents an example of horizontal transmission of genetic information between plants and animals.