The toxicity of the glyphosate-based herbicide formulation Roundup Transorb^® was determined for Poecilia reticulata (guppy). The mean lethal concentration (LC₅₀) for 12, 24, 48, 72 and 96 h of R. Transorb^® were 11.2, 8.5, 6.5, 6.1 and 5.6 μl/L, respectively, indicating that this species is very sensitive to this herbicide. Our recent study showed its genotoxicity to P. reticulata at low-level exposures of 1.41, 2.83, 4.24 and 5.65 μl/L which increased the frequencies of micronucleus and DNA damage in peripheral erythrocyte cells. Toxicity bioassays were conducted to evaluate the effects of sublethal concentrations of herbicide in the acute treatment for 24 h. Fish exposed to the herbicide showed behavioral changes such as aggressiveness and loss of sense of direction. Several different pathological alterations were observed in the liver, such as vacuolization, cytoplasmic degeneration and hyperemia. Therefore, results confirm the risks this herbicide formulation poses to aquatic organisms.