After penetration into its snail host the trematode not only has to find a suitable environment, it also has to circumvent the activities of its host's internal defence system to survive and to develop normally. Immunological compatibility of snails and trematodes is dependent on the capability of the parasite to evade and/or suppress the defence activities of its host. The possible role of physicochemical properties of the surfaces of the parasite and the host defence cells in evasion of host responses remains unknown; a role is indicated for molecular mimicry in the miracidium and the young mother sporocyst and for masking in the cercaria. Strong evidence exists that trematodes secrete substances which suppress anti-trematode activities of snail haemocytes.