We have used a styryl dye FM 1–43 that cannot cross membranes passively to evaluate membrane vesicle internalization processes from the heptalaminate surface membrane in schistosomula and adult Schistosoma mansoni. The process of dye internalization was faster in adult worms than in 3 h skin or 24-h-old mechanically transformed schistosomula. Dye internalization and vesicle formation were inhibited at low temperature which suggests that the process is energy dependent. Dye internalization was observed to be a result of vesicle formation. This process is impaired by primaquine independently of the parasite stage. The specific Ca2+ chelator EGTA affected dye internalization at a concentration of 5 mm. When parasites were labelled in the presence of genistein (a tyrosine kinase inhibitor) uptake of the dye was blocked in 24-h-old schistosomula. However, only a partial blockage of the dye internalization was seen in adult worms. These results suggest that dye internalization mediated by tyrosine kinase signalling is developmentally regulated.