ABSTRACT Kunc, F., 1991. Organic substrates and microbial conversion of herbicides in soil. Presence of organic compounds in soil is one of the main factors that determine the activity of microorganisms including their capability of conversion of xenobiotics. The effect manifests itself at various levels of the system's complexity: from the subcellular level (genetic, enzymatic) to that of microbial communities in natural environments. This review deals with final consequences of the effect of native and added organic substrates on the decomposition of herbicides and with some mechanisms involved in these processes. The role of organic compounds in the control of the synthesis and activity of degradation enzymes, in cometabolic processes and in the history of nutrition of microbial associations with respect to their adaptation is documented. Limitation of the decomposition due to adsorption and formation of complexes of the herbicides with the organic substrate as well as due to the effect of the conditions of soil environment are also described. Possibilities of using the knowledge to control the microbial degradative activity are suggested with the aim to stimulate the removal of undesirable xenobiotic residues from soil environment.