Most herbicides are complex molecules and their decomposition often requires several different chemical conversions, each mediated by a variety of organisms. Until now the detailed investigation of the ecology of such organisms has been hampered because of the need to have sufficient quantities of h e rbicide degradation products for inclusion in the selective media r equir ed for their enumeration. The rationale behind the approach to the pre sent work is that each of the chemical conversions performed by microbes will be catalysed by enzymes. The quantitative detection of the genes coding for these enzymes in soil microbial communities should indicate whether or not a soil contains organisms able to degrade a herbicide and, if it does, whether the rate of degradation will be relatively fast or slow .,