The present study was designed to delineate the effect of Giardia duodenalis in malnourished and renourished BALB/c mice. Control and renourished mice were fed with a standard pellet diet while malnourished mice were fed with a low protein (4.3 %) diet both before and after being challenged orally with actively growing G. duodenalis trophozoites. It was observed that malnourished mice had a greater severity and longer duration of Giardia infection compared with renourished mice. These malnourished mice also had less body mass but higher cyst and trophozoite counts. Malnourished mice infected with Giardia had significantly decreased level of total serum proteins, albumin, globulins, hemoglobin, leukocyte, and differential leukocyte counts compared with renourished mice. From the data it is concluded that protein malnutrition profoundly affects the anthropometric and physiological parameters of the body indicating greater susceptibility and severity of the disease.