S-metolachlor is one of the most frequently used herbicides worldwide. However, toxicity studies of this herbicide to aquatic organisms are scarce. In the present study, two experiments were conducted to test the effects of S-metolachlor on common carp, one of the most economically important fish species, with a distribution throughout the world: (1) 96 h acute semi-static toxicity test, aiming to determine LC₅₀; (2) a subchronic semi-static test that lasted 28 days, in which juvenile carp were exposed to 3%, 8%, and 25% of previously determined LC₅₀—0.5 mg·L⁻¹, 1.4 mg·L⁻¹, and 4.1 mg·L⁻¹, respectively. Several biomarkers were employed to assess fish responses to toxicants. Blood biochemistry analysis and nuclear alterations of erythrocytes did not show any difference among experimental groups. Semi-quantitative histopathological analysis revealed mild alterations in the gills and liver, where oedema of secondary epithelium of gills and leukocyte infiltration in liver were significantly higher in fish exposed to 1.4 mg·L⁻¹ and 4.1 mg·L⁻¹. Histopathological indices in liver, as well as the total histopathological index, also showed significantly higher scores in the same groups. Bioconcentration factors of S-metolachlor ranged from 3.2 to 9.4, depending on the experimental group.