Mahua oil cake, a derivative from the plant Bassia latifolia, is extensively used as fish toxicant cum pond manure in inland aquaculture systems of India. Saponin, one of the haemolytic components of mahua oil cake, has a lethal effect on fishes and other organisms. In the present investigation, Heteropneustes fossilis-an air-breathing catfish and Cyprinus carpio-a carp, were treated with mahua oil cake and their haematological indices were evaluated. The effect was fatally critical with gradual and complete shrinkage of erythrocytes, followed by ultimate haemolysis of cells, and significant reduction of the values of haematocrit, haemoglobin concentration and red cell count. © 1986 Indian Academy of Sciences.