An Ac/Ds-based two-element transposon tagging system has been introduced into carrot. F-1 progeny containing both the 35S-Ac-transposase gene (35S:TPase) and the Ds element were derived from crosses between 35S:TPase- and Ds-bearing parents. While excision of Ds was not detected in any F-1 plants carrying both 35S:TPase and the Ds element, calli initiated from these F-1 plants had the Ds element excised, indicating Ds transposition. Reverse transcriptase-PCR analysis revealed that the 35S:TPase gene was expressed in both F-1 plants and calli, and that introns 1, 2, and 3 were spliced correctly. Although intron 4 was also spliced correctly in calli, incorrectly spliced intron 4 was detected in F-1 plants. Sequence analysis of incorrectly spliced reverse transcriptase-PCR products demonstrated the presence of a cryptic intron donor site within intron 4 of the 35S:TPase transcript. This probably competed with the proposed intron donor site during maturation of the major 35S:TPase transcript. These results suggested that the major transcript of 35S:TPase was incorrectly processed and, consequently, that the Ds element was reversibly inactivated in the somatic tissues of carrot plants, whereas this inactive Ds element was remobilised during tissue culture, where the 35S:TPase transcript was spliced correctly. These observations point to an important role for tissue-specific 35S:TPase transcript processing for successful transposition of Ds in carrot. Therefore, successful processing of the 35S:TPase transcript in carrot callus may indicate strategies to increase Ac transposition in other tissues.