ABSTRACT The present study investigated the toxic effects of 1‐butyl‐3‐methylimidazolium bromide ([C 4 mim]Br) in zebrafish livers after exposure to 5–40 mg/L of [C 4 mim]Br on days 7, 14, 21, and 28. The results showed that low levels of [C 4 mim]Br exposure activated the superoxide dismutase (SOD) activity during early periods of exposure. However, high concentrations (20–40 mg/L) of [C 4 mim]Br caused the inhibition of SOD, which, accordingly , led to excess reactive oxygen species and malondialdehyde. The present results indicate that [C 4 mim]Br causes oxidative stress in the livers of both male and female zebrafish. Additionally, the DNA damage revealed that [C 4 mim]Br has a genotoxic effect on zebrafish livers, even when the concentration of [C 4 mim]Br is low (5 mg/L). The DNA damage was demonstrated by quantifying the DNA strand breakage.