The present study was undertaken to investigate the genotoxicity of amoxicillin in rainbow trout (Oncorhynchus mykiss) erythrocytes. The DNA damage was measured as the percentage of DNA in comet tails and micronucleus formation in erythrocytes of fish exposed to different concentrations of amoxicillin (80, 160 and 320 mg/kg bw). Cyclophosphamide was used as a positive control (4 mg/L). Blood erythrocytes were sampled on day 3, 6 and 10 of exposure. It was obtained that, amoxicillin induced a significant increase (p<0.001) in the percentage of DNA in comet tails as well as micronucleus frequency at 320 mg/kg bw treatment group when compared with negative control group at all exposure times. The highest DNA damage was found on day 6 of exposure with both micronucleus test and comet assay. The results of the study indicated that, amoxicillin has genotoxic effects on fish at high concentrations.