In a short feasibility study, we detected the induction of nuclear anomalies in the peripheral blood erythrocytes of fish by the genotoxic compound ethyl methanesulphonate. Exposure of the eastern mudminnow Umbra pygmaea to the compound gave rise to nuclear segments resembling the particles found in mammalian micronucleus tests, but also to irregular Feulgen-positive structures in the cytoplasm. Further studies with fish erythrocytes are recommended for investigating whether the monitoring of nuclear anomalies is a suitable and rapid method for the detection of genotoxic compounds in the aquatic environment.