The maize transposable element Activator (Ac) has been shown to be active in a number of dicots, including Arabidopsis thaliana, whose small genome and short generation time have favored its wide adoption as a model organism for molecular genetic approaches to plant physiology and development. Using the Ac element and several bacterial and plant marker genes, we have devised a versatile system for identifying plants in which a transposon has excised and reinserted elsewhere in the genome. The transposons have been designed to facilitate the identification of insertions downstream of promoters and in the vicinity of enhancers by the inclusion of a beta-glucuronidase (GUS) gene either lacking a promoter or having a minimal promoter sequence. The system permits the transposon and the source of transposase to be maintained either stably in separate plants or in the same plant. Plants in which transposition is occurring can be identified by the frequent somatic activation of the GUS gene. The herbicide chlorsulfuron is used as a selective agent to identify progeny plants in which the transposon has excised from its original insertion site within a chlorsulfuron-resistant acetolactate synthase gene. Additional selectable markers permit the identification of plants containing a transposed element, but lacking transposase. Here we describe our initial characterization of the system and demonstrate its reliability and efficiency in identifying plants with transposed elements.