The susceptibility of B. alexandrina to infection with S. mansoni miracidia varies due to biological & ecological parameters. In case of snail infection in less than 4 or 5 months productivity doubled between 1 and 2 miracidium, but with more than 2 miracidia with practically no increase in productivity. In more than 5 months, there no was discernible difference in productivity between monomiracidial and polymiracidial infections. The production of cercariae was characterized by very irregular daily emissions and by the alternation of periods of high and low emissions, which gave rise to a production rhythm.