The present study was designed with the aim to study the modulation of giardiasis with natural intervention, i.e., Lactobacillus casei--a probiotic, in a murine model. Probiotics are viable microorganisms having a beneficial effect on the prevention and treatment of specific pathological conditions. Various dairy products such as curd, lassi, and kulfi were employed particularly to isolate live organisms (probiotics). Out of five isolated strains only one was found to be similar to a standard strain--Lactobacillus delbrueckii. Experimentally, it was observed that oral feeding of isolated strain A, 7 days prior to inoculation with Giardia lamblia trophozoites, was not as efficient as L. casei in eliminating Giardia infection from BALB/c mice. Interestingly, it was observed that L. casei fed 7 days prior to Giardia infection was more effective and efficient in eliminating the infection from mice. On histological examination, it was observed that probiotic-fed mice had less atrophied villi and infiltrating cells in the small intestine compared to the control. Ultrastructural studies by scanning electron microscopy further confirmed the protection of the mice dosed with L. casei 7 days prior to Giardia infection and also when simultaneously infected with Giardia. It is felt that probiotics, particularly L. casei, modulate Giardia infection by minimizing or preventing the adherence of Giardia trophozoites to the mucosal surface, suggesting that probiotics offer a safe and effective mode to prevent and treat Giardia infection.