AbstractThe unicellular green algaChlamydomonas reinhardtiiwas used as a model organism in studies of the mode of action of the oxyacetamide herbicide mefenacet. An algal strain with a decreased sensitivity towards mefenacet was obtained by successively increasing the herbicide concentration in the culture medium from 10 to 50 μM over 9 weeks. Similarly, a second strain was adapted to 10 μM metazachlor and a third one to 20 μM metolachlor. The resistance patterns of these three strains were studied with a selection of herbicides representing different modes of action. In the mefenacet‐ and metolachlor‐adapted strains, clear cross‐resistance was only obtained between the oxyacetamide mefenacet and the chloroacetanilides alachlor, butachlor, metazachlor and metolachlor. The resistance factors (I50in resistant/I50in sensitive algae) were between 4 and 50. Little or no resistance was obtained when herbicides with other modes of action were studied (resistance factors between 0.5 and 4). The situation was less clear in the metazachlor‐adapted strain, but here also strong cross‐resistance was seen with other chloroacetanilides and mefenacet (resistance factors between 25 and 40). This strain was, however, significantly cross‐resistant to some herbicides with other modes of action.Investigations of the possible resistance mechanism in the mefenacet‐adapted strain have shown that neither uptake nor metabolism can explain the observed resistance behaviour. The lipid composition and the pattern of soluble amino acids were unchanged. It is concluded that oxyacetamides and chloroacetanilides have the same site of h
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