Insertional mutagenesis is a basic genetic tool that allows for a rapid identification of the tagged genes responsible for a particular phenotype. Transposon and Agrobacterium -mediated DNA integration are the most commonly used biological mutagens in plants. The main drawback of these technologies is the relatively low frequency of mutations, as compared to those induced by conventional chemical or physical agents, thus limiting the use of insertional mutagens to the generation of large mutant populations in few genetic backgrounds. Recent improvements in Agrobacterium -mediated transformation efficiency and an increasing repertoire of transformation vectors available to the research community is making this type of mutagen very attractive for individual laboratories interested in the studies of mutations in particular genetic backgrounds. Herein, we describe a simple yet robust Arabidopsis transformation procedure that can be used to generate large numbers of insertional mutants in Arabidopsis thaliana . Using this protocol, transformation efficiencies of up to 5% can be achieved.