The maize transposable element Activator (Ac) has been shown to be active in a number of dicots, including arabidopsis [Arabidopsis thaliana (L.) Heynh.], tobacco (Nicotiana tabacum L.), tomato (Lycopersicon esculentum Mill.), potato (Solanum tuberosum L.), and aspen (Populus tremuloides Michx.). However, no information is available on somatic transposition in any plant during several years of growth and development. It is not known how transposition affects genetic variability among vegetative parts that have developed during a long period of growth. In order to explore the possibility of using somatic Ac transposition for gene tagging and mutagenesis in fruit trees, a derivative of the maize Ac transposable element was introduced into `Duncan' grapefruit (Citrus paradisi Macf.) by Agrobacterium tumefaciens (Smith & Towns.) Conn.-mediated stable transformation. Genetically identical 4-year-old sibling trees were established by grafting one of the transformants on Troyer citrange [Citrus sinensis (L.) Osbec. × Poncirus trifoliate (L.) Ras.] rootstocks. We demonstrated that the Ac element was active upon transformation in citrus (Citrus L.) trees and that transposition can create genetic variability among tree siblings and among leaves collected from different parts of the same tree. Ac was still active among propagated plants 4 years after transformation, clearly indicating that it is capable of maintaining itself in citrus trees for a long period of time. The observation of different integration patterns in different parts of the same tree and within tree siblings originating from the same transformant suggests that an Ac-based mutagenesis system could be very useful in creating somatic mutations in citrus trees.