Surface tegumental membrane sugars of cercariae, schistosomula, and adult-stage Schistosoma mansoni were examined using concanavalin A and wheat germ agglutinin. The cercarial stage showed no reactivity for all the lectins tested. Schistosomula reacted strongly with concanavalin A only. Treatment of living adults with neuraminidase did not alter the fixation of Con A or wheat agglutinin. Induction of lateral translational motion of intramembrane particles in the outer tegumental membrane was revealed by freeze-fracture technique after Con A and WGA binding. The constant formation of aggregates of two preexiting intramembrane particles was observed and a sliding of these complexes to the tegumental spines was detected. The significance of these findings is discussed in relation to the membrane characteristics and to a possible ability of the worm to modulate its membrane.