This investigation was planned to disclose the capability of the well- known drug Na- metamizole in inducing clastogenic effect (s).             To achieve such a purpose four doses correspond to one, two, five; and ten folds of the daily therapeutic dose were tested. Three different organisms i.e. mice (Mus musculus, 2n= 40); (Tilapia zilli, 2n= 42); and catfish (Clarias gariepinus, 2n= 56) were used employing the following genotoxic bioassays: 1- Analysis of chromosomal abnormalities in mice bone- marrow cells; 2- Micronucleus test of micronucleated polychromatic erythrocytes in mice bone- marrow cells; and 3- Analysis of  micronucleated erythrocytes in two different genera of fish. Indoxane (cyclophosphamide, CP) was used as positive control.             The results revealed that Na- metamizole was proven to be highly positive clastogenic agent at all tested doses upon mice as well as fish genomes.             The work recommends that this drug should be omitted from use in Egypt.