The action of the parasitism by the digeneic trematode Tanaisia bragai over the mortality, reproductive function, growth rate, calcium concentration in shells and oxygen consumption of its intermediate host, the snail Subulina octona were experimentally studied in infected and in infected snails maintained under laboratory conditions. A significant increase in mortality was observed in the infected snails. The alteration in the reproductive function evaluated through the egg production, suggested parasitic castration of infected snails. The Student's «t» test showed that there was a significant difference in relation to the shell weight and calcium concentration in the shells of infected and uninfected snails and there was no significant difference in relation to shell lenght, growth rate and oxygen consumption in the infected and uninfected snails.