the aim of this research was to evaluate the ecotoxicological effect of methamidophos using two formulations of different toxicological class [monofos？, class ia (extremely dangerous) and tamaron？, class ib (highly dangerous)] on four non-target aquatic organisms: bloodworm chironomus calligraphus (goeldi, 1905) (diptera: chironomidae), black sea urchin tetrapygus niger molina, 1782 (echinodermata: arbaciidae), neon tetra paracheirodon innesi (rabout, 1940) (osteichthyes: characidae); and rainbow trout oncorhynchus mykiss (walbaum, 1792) (osteichthyes: salmonidae). both methamidophos formulations evidenced a high risk effect on the aquatic environment, finding effects on larvae of c. calligraphus (class ia, lc50 at 48 h = 1.32 mg a.i. l-1 and class ib, lc50 at 48 h = 4.5 mg a.i. l-1), on fertilization of t. niger (class ia, ic50 at 1 h = 1423 mg a.i. l-1 and class ib, ic50 at 1 h = 608 mg a.i. l-1), on p. innesi (class ia, lc50 at 96 h = 20.56 mg a.i. l-1 and class ib, lc50 at 96 h = 10.13 mg a.i. l-1) and o. mykiss (class ib, lc50 at 96 h = 19.12 mg a.i. l-1). the sequence of sensibility to methamidophos in both formulations was: c. calligraphus > o. mykiss ≈ p. innesi > t. niger. in addition, two sublethal effects were evaluated on p. innesi, immobilization and strange swimming, and finally an increment of opercular movement in o. mykiss. risk quotients (rq) indicated in all cases a high risk of methamidophos towards the aquatic environments.