The cercaria of Isthmiophora melis is relatively specific to suitable intermediate hosts, amphibia and fish, as early as the stage of attachment. Attachment responses are maximally stimulated by intact amphibia, whereas isolated frog tissues have a reduced efficiency. Human urine contains attachment triggering substances. Its effectiveness is reduced, when carbonate systems are removed, and may be restored by addition of carbonate. Aqueous solution systems of dissolved CO2 and H2CO3/HCO-3 stimulate attachment responses, especially at pH 6.0-7.5. The attachment response to frog skin surface seems to be triggered by the buffering system, which uses carbonate solution equilibria as important components.