An unusual property of schistosomes is their capacity to survive intravascularly for many years in the face of an ongoing antiparasite immune response by the infected host. This ability to evade the immune system appears to be due to several unusual parasite adaptations occurring soon after infection is initiated. Foremost amongst these are processes which result in reduced surface antigenicity and the development of a tegument intrinsically resistant to immune damage. The unusual ability of the schistosome surface membrane to escape immune recognition and damage suggests that attempts to vaccinate against this important parasite either should be directed against very early schistosomula which have yet to achieve the refractory state, or involve stimulation of effector mechanisms, such as killing by activated macrophages, which do not require recognition or interaction with parasite surface antigens.