Purpose:Cypermethrin (class II) belongs to pyrethroid insecticides which have been in wide use to control various pests including moth pests of cotton, fruit and vegetable crops. This study was aimed to investigate the effects of cypermethrin on haematology of female rats and their modulation by co-administration of sodium selenite. Methodology:Four equal groups of female rats were made (n=20). Group 1 received 1 ml corn oil, Group 2 was administered 55 mg/kg/b.w of cypermethrin, Group 3 was provided with 1 ppm of sodium selenite and Group 4 was given both cypermethrin and sodium selenite. Haematological analysis of blood samples was performed employing Sesmex Kx-21 (Japan). Results:Red blood cell (RBC) count, haemoglobin concentration (Hb) and hematocrit (Hct) were significantly reduced (P Conclusion:Hence, it can be concluded from this study that cypermethrin administration can cause hematological alterations in rats. Co-administration of selenium provided protection against cypermethrin toxicity to some extent.