Homologous recombination has been used for two decades to target insertions into cloned genes in bacteria and yeast, and more recently has become a routine method of gene inactivation in mammals. Arabidopsis is one of several multicellular model organisms (along with Drosophila, Caenorhabditis and zebrafish) in which mechanisms controlling development have been studied. Previously, traditional genetic methods have been used, as targeted disruption by homologous recombination has not been successful in any of these organisms. We have now successfully disrupted the AGL5 MADS-box gene in Arabidopsisby homologous recombination, providing a useful tool for future analyses.