AbstractFour reference toxicants (Cr6+, Cu2+, Zn2+, phenol) and 9 herbicides (imazamethabenz, 2,4‐dichlorophenoxyacetic acid, picloram, glyphosate, bromoxynil, metolachlor, diquat dibromide, hexazinone, cyanazine) were appraised using both the microplate and flask assay Ninety‐six hour EC50s determined withSelenastrum capricornutumas the indicator species essentially demonstrated good intermethodological data concordance for all chemicals, with the exception of diquat dibromide, whose phytotoxicity in the microplate assay (EC50= 4.9 μg · L−1) was nearly 7 times that of the flask assay (EC50= 34.2 μg · L−1). Comparisons with other data in the scientific literature relating to similar herbicides with the same or different green algal indicator species appeared to corroborate the overall data obtained in our study. More than 4 orders of magnitude separated the most toxic (cyanazine, flask, and microplate EC50s of 17.6 and 16.9 μg · L−1, respectively) and the least toxic (imazamethabenz, flask, and microplate EC50s of 89.1 and 91.1 mg · L−1, respectively) herbicides. The biprocedural phytotoxicity comparison described in this work suggests that the simpler algal microplate assay can be an appropriate alternative to the flask technique to evaluate the algal growth inhibition eff