A monoclonal antibody directed against the circumsporozoite protein on the surface of Plasmodium berghei sporozoites inhibited sporozoite motility in vitro. These immobilized sporozoites could adhere to but not invade target cultured cells. Other sporozoite-immobilizing agents also inhibited sporozoite invasion into cultured cells and did not prevent sporozoite adherence. These results indicate that sporozoite invasiveness is associated with sporozoite motility. Thus, the immobilizing effect of this antibody could explain its functional activity against sporozoite invasion in vivo.