Giardia sp. trophozoites were isolated directly from the small intestines of rats and permitted to attach to polystyrene Petri dishes incubated at 37 C. Attached trophozoites were treated in vitro with various agents which inhibit cell motility (cytochalasin-B, low Ca++, colchicine) and metabolism (iodoacetic acid, 2,4-dinitrophenol) and chemotherapeutic agents (quinacrine-HCl, metronidazole). Trophozoite attachment was sensitive to cytochalasin-B, low Ca++, iodoacetate, and quinacrine-HCl. Contractile proteins (actin, myosin) have been demonstrated previously in the periphery of the ventral disc. The effect of cytochalasin-B and low Ca++ concentrations on trophozoite attachment is consistent with the suggested role of contractile proteins in the mechanism of attachment. The effect of iodoacetate suggests that energy for attachment is derived from glycolysis. The effect of quinacrine-HCl on attachment was rapid (less than 10 min with 10.0 micrograms/ml). Its mode of action on attachment is not understood.