Summary: The transposase (TPase) of the maize transposon Activator (Ac) accumulates in the nuclei of maize endosperm and transfected Petunia protoplasts, where it aggregates into rod‐like structures about 2 μm in length. In petunia protoplasts the amount of TPase aggregates increases with the strength of the promoter fused to the Ac‐coding region. The excision frequency of a Ds element, however, does not increase proportionally. The data suggest that the aggregated TPase is not responsible for the mobilization of the Ds element, but rather is a transpositionally inactive form of the protein. In contrast to the full‐length TPase, a functional, N‐terminally truncated TPase derivative is inefficiently transported into the nucleus at high expression levels and aggregates predominantly in the cytoplasm. Accordingly, the N‐terminus of the TPase is involved in nuclear localization and/or aggregation.