In the cercarial and schistosomal stages of the life cycle of the trematode Schistosoma mansoni, acetylcholinesterase occurs as two principal molecular forms (both globular), present in approximately equal amounts, with sedimentation coefficients of 6.5 S and 8 S. The 6.5 S form is solubilized by bacterial phosphatidylinositol-specific phospholipase C from intact schistosomula. It is thus located on the outer surface of the schistosomal tegument and is most probably analogous to the glycosylphosphatidylinositol-anchored G(2) form of acetylcholinesterase found in the electric organ of Torpedo, on the surface of mammalian erythrocytes, and elsewhere. Both forms are fully solubilized by the non-ionic detergent Triton X-100. Upon passing such a detergent extract over a heparin-Sepharose column, only the 8 S form was retained on the column. The bound acetylcholinesterase could be progressively eluted by increasing the salt concentration, with approx. 0.5-0.6 M NaCl being needed for complete elution. Selective inhibition experiments carried out on live parasites using the covalent acetylcholinesterase inhibitor echothiophate (phospholine), which does not penetrate the tegument, selectively inhibited the 6.5 S form, but not the 8 S form, suggesting an internal location for the latter. Monoclonal antibodies raised against S. mansoni acetylcholinesterase also distinguished between the two forms. Thus monoclonal antibody SA7 bound the 6.5 S form selectively, whereas SA57 recognized the 8 S form. The selective binding of the 8 S form to heparin suggests that, within the parasite, this form may be associated with the extracellular matrix of the musculature.