ACCORDING to the UN Food and Agricultural Organisation production year book of 1974, 27×106 kg of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) were consumed in 1973, making this the most extensively used of all pesticides. One desirable characteristic of 2,4-D, apart from its efficacy as a herbicide, is its lack of persistence in the environment1, due to degradation by a variety of microorganisms. We have argued that in view of the isolation of plasmids encoding the degradation of naturally occurring aliphatic and aromatic compounds2, there exists a class of plasmids which specifically encode the degradation of many man-made derivatives of these compounds, in this instance 2,4-D. Using segregation and transfer analyses we have demonstrated that a plasmid or plasmids is involved in 2,4-D degradation. Clearly bacteria have evolved which can degrade many different pesticides; our results indicate that plasmids may have an important role in this evolution.