A laboratory experiment was performed to study the role of microorganisms in producing the non‐extractable residues by anaerobically incubating cyanazine (2‐[[4‐chloro‐6‐(ethylamino)‐1,3,5,‐triazin‐2‐yl]‐amino]‐2‐methoxy‐l‐methylethyl)acetamide] and metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methoxy‐l‐methylethyl)acetamide] in culture media that had been inoculated with sewage bacteria from anaerobic sludge. Based on the gas chromatographic analyses of extracts from the degradation, adsorption, and abiotic controls for the parent herbicide and its possible metabolites, this study provided the first direct evidence that bacterial biomass, rather than metabolism, was mainly involved in the formation of bound residues with cyanazine. Anaerobic bacteria appeared to be incapable of forming bound residues with metolachlor. The common phenomenon of age‐dependent extractability for bound residues was observed with cyanazine. These results imply that bacteria may adsorb pesticides selectively with preference for certain chemical structures over others.