The study describes the in vivo activity of Lactobacillus casei in malnourished Giardia lamblia-infected BALB/c mice. By experimentation, it was found that daily administration of the probiotic 7 days before inoculation with Giardia trophozoites in malnourished mice efficiently reduced both the severity and duration of giardiasis. More specifically, excretion of Giardia cysts and trophozoites counts were reduced, while faecal lactobacilli counts increased significantly in probiotic-fed malnourished mice, compared with control mice. Interestingly, it was also observed that oral feeding of the probiotic to malnourished mice abrogated all the anthropometric and biochemical anomalies. Histologically, morphological and cellular alteration of microvillus membrane integrity revealed that probiotic administration ameliorated the mucosal damage in malnourished, probiotic-inoculated, Giardia-infected mice compared with the severe microvillus atrophy, œdematous and vacuolated epithelial cells, and ileitis in malnourished Giardia-infected mice. The results clearly show the antigiardial effect of the probiotic in vivo by modulating the gut cells to inhibit the colonization and multiplication of Giardia trophozoites, thus reducing the severity and duration of murine giardiasis.