Resumo. The glyphosate-based herbicides such as Roundup®, are widely used around the world. Data on the abilityof these toxic compounds have to interfere in the DNA caused mutations are important for monitoringnatural populations. The aim of this study was to evaluate the mutagenicity of Roundup® by micronucleustest in fish (Astyanax altiparanae), complementing with the toxicity test for inhibition of root growth inAllium cepa. Fish were exposed to 5, 7.5 and 10 μl l-1 for 24, 48, 72 and 96 hours; after which a bloodsample was taken for analysis of micronuclei in erythrocytes and nuclear aberrations: lobed, blebbed andnotched. The results obtained with the micronucleus test showed that Roundup® is not mutagenic in theorganisms tested (A. altiparanae) for different concentrations and exposure times (P > 0.05). The testof inhibition of root growth, using onion bulbs exposed to the same concentrations used in fish, provideimportant additional information. In this test, Roundup® proved to be toxic at the concentration of 10 μll-1 (P = 0.003). Although Roundup® does not have mutagenic effects in the present study, was impossibleto discard its toxic effect, because, in addition to inhibition of root growth in onion, it proved lethal tofish when exposed to higher concentrations (20 μl l-1). Further studies are needed, mainly using chronicexposures, to investigate the real effect of the Roundup® and glyphosate in the body of organisms exposedto these.