Nearly a decade ago, Dr. Donald S. Robertson first described the Mutator system in Zea mays. Robertson's Mutator maize lines contain a highly mutagenic transposable element system. Mutator induces new germinal mutation at a frequency 50-fold above the background level. Approximately 40% of the mutants induced are somatically unstable (mutable). Transposable elements, called Mu elements, have been implicated in the mutagenic activity of Mutator;Genetic analyses of an inbred lineage of a mutable, Mutator -induced mutant of A1, a1-Mum2 were carried out. Conditions which led to the loss of somatic mutability and germinal activity were investigated. A putative functional change in Mutator regulation is described;Molecular analyses of the Mutator transposable elements present in the a1-Mum2 inbred lineage was conducted. The types of Mu elements present in this line, their modification state, and copy number are related to the regulation of the phenomena, somatic mutability and germinal activity;A survey of maize lines for the presence of a mutator activity similar to that of Robertson's Mutator system was performed. Additionally, maize lines from diverse geographic origins were analyzed by Southern hybridization techniques for the presence of Mu -homologous DNA sequences. The origin of Robertson's Mutator system is considered in the light of these results.