In tomato (Lycopersicon esculentum Mill.), extensive variation can be observed in genes conditioning many plant functions including disease resistance, plant development, and fruit, leaf and hypocotyl pigmentation. Genes exist that confer resistance to viruses, nematodes, bacteria and fungi. There is great value in developing a facile system for isolating such genes, and we are attempting to create a tomato gene isolation system based on insertional mutagenesis with the maize transposon Ac. Our chosen target genes (Cf-2, Cf-4, Cf-5 and Cf-9) confer resistance to specific races of the fungus Cladosporium fulvum Cooke, the causal agent of leaf mould. We have used classical and molecular techniques to map three of these genes, correcting errors in previous reports. Ac-carrying T-DNA constructs have been developed to facilitate monitoring of Ac activity in tomato, where Ac is extremely active. These constructs have been used in plant transformation experiments, and over 100 tomato lines carrying Ac have been created. More than 20 T-DNAs carrying Ac have been localised on the tomato genetic map by testing linkage to RFLP loci. This was carried out to exploit the preference of Ac for transposition to linked sites in our tagging strategy.