Effect of plumbagin, a phytochemical, on the haemocytes ofDysdercus koenigii was studied after topical application. Scanning electron microscopical studies showed deformity in surface morphology in almost all the 5 types of haemocytes categorized in the bug, especially that of granular haemocytes and plasmatocytes, which are devoid of their filopods in the treated insects. The fat droplets of adipohaemocytes shrink while oenocytoid is affected to a lesser degree. Plasma membrane of all the cells becomes fragile leading to a gradual loss of cytoplasm and ultimately only a few interconnected cytoplasmic strands are left. Ultrathin sections revealed a highly vacuolized condition and disintegrating organelles which pass out of the cells due to ruptures of very thin plasma membrane. Total and differential haemocyte counts performed after 24 and 48h of treatment showed a drastic reduction of all the types i.e. the prohaemocytes disappear from blood, the number of granular haemocytes and plasmatocytes declines while oenocytoids and adipohaemocytes comprise the major part of counts. Because of the damages caused to haemocytes and the suppression of filopodial elongations of plasmatocytes and granular haemocytes (the types that are active in defense mechanism), it can be inferred that cellular defense reactions ofDysdercus koenigii are reduced after plumbagin treatment.