It has been shown that in both Physa fontinalis (L.) and Physa acuta Drap. there is a difference in sensitivity of different parts of the body to contact with Glossiphonia complanata (L.). Contact with the mantle-fringes was found to give the highest percentage of leech-avoidance reactions. These fringes are also the parts most sensitive to mechanical stimulation. Ph. fontinalis (L.) and Ph. acuta Drap. show considerable difference in their reactions to contact with Gl. complanata (L.), Ph. fontinalis (L.) being by far the more sensitive. In addition to Gl. complanata (L.), leech avoidance reactions of Ph. fontinalis (L.) were also produced by Hirudo medicinalis (L.), Hemiclepsis marginata (O.F.M.) and Helobdella stagnalis (L.) The characteristic leech-avoidance reaction of Ph. fontinalis (L.) could also be produced by solutions of NaCl, NaBr and NH4Cl and it was shown that it is the sodium or ammonium ion and not the anion that is responsible for the reaction. Boiling of extracts of Gl. complanata (L.) gave a precipitate, presumable a protein, to which Physa fontinalis (L.) responded with the leech-avoidance reaction. The pH of the extracts did not influence the activity of the precipitate, so that it may be concluded that ions bound to the protein by saltformation are not concerned in the reaction.