The effect of three ferric chloride concentrations (100, 200, 300 mg/l) on the acid-alkaline balance of haemolymph and haemoglobin content in Planorbarius corneus under normal conditions and in the case of infection with parthenites and larvae of Echinoparichium aconiatum was examined. In the medium with an effective concentration of ferric chloride, all snail specimens proved to have acidified haemolymph, by 40-50% in infected samples and 80-85% in uninfected ones. Statistically reliable differences in the haemoglobin content in the haemolymph of infected and uninfected specimens between the control and test media were absent, that proves a weak toxic effect of ferric ions on P. corneus.