The occurrence of bacteria capable of accumulating and/or degrading methidathion was analyzed in various samples: methidathion-resistant mites, soils and leaves previously treated with the pesticide, and sewage muds. Twenty-nine bacterial strains from laboratory collections were also tested. Results suggest that this degradation is a very scarce phenomenon. The natural microbial population from greenhouse soil, rose leaves, and mites lacked microorganisms able to perform such degradation. Nevertheless, a gram-positive strain belonging to the species Bacillus coagulans was able to produce at least four metabolites from [14C]methidathion, unusable as sole source of carbon and energy in a mineral medium supplemented with glucose. Since desmethyl methidathion was one of the major metabolites, it is suggested that one of the possible pathways of the bacterial degradation of this pesticide could be demethylation via a glutathione transferase. The presence of plasmid DNA in this strain was not positively demonstrated.