Freshwater microalgae are primary producers and cosmopolitan species subjected to the effects of herbicides. In this work, thein vitroalgal growth inhibitory effects of 11 agrochemicals (9 herbicides, 1 metabolite, and 1 safener) were quantified. Chemical compounds were applied singly and in specific mixtures. Three species were used in axenic condition: the green algaDesmodesmus subspicatus(Chodat), the diatomsNitzschia palea(Kützing) W. Smith andNavicula pelliculosa(Kützing) Hilse. When exposed to single compounds,N. paleaandN. pelliculosawere only sensitive to atrazine/desethylatrazine and the safener benoxacor (BE), respectively.D. subspicatuswas equally sensitive to four herbicides including atrazine and its metabolite and significantly more sensitive to iodosulfuron-methyl-sodium (IODO). The mixture of these five compounds induced a significantly higher growth inhibition of about 1.5-fold than IODO alone, which could be attributed to the four other herbicides. The mixture of all compounds was twofold less toxic than IODO onD. subspicatus. A halogen atom is present in IODO as in the herbicides to which the safener BE − known to induce glutathione-S-transferases − is associated in agrochemical preparations. We then showed that IODO was less toxic when combined with non-toxic concentrations of BE. These results indicated that the toxicity of the most active herbicide studied was decreased by a non-herbicide compound present in agrochemical formulations of other herbicides. These results suggest the importance to take into account the chemistry and the mechanisms of action for each compound in a risk assessment approach of a complex mixture.