The authors have infected 127 B. glabrata by 2 miracidia of S. mansoni, either on the Same day, or introducing a second miracidium after 3, 7 and 16 days. 1 All the groups of planorbid snails had a low percentage of positivity (31 to 41 %). 2 The first cercarial emissions, in the 4 groups were scattered in the time, the delay being in correlation with the second infection. 3 In the snails reinfected after 3 and 7 days occured the highest emissions, those with the simultaneous double infection or second infection delayed to 16 days, had the lesser emissions. 4 An interval cycle of about 3 weeks was discovered for the highest emissions. All those phenomena are probably due to competition between the sporocysts born from both miracidia. Moreover, evident reduction of the fecundity in the positive snails were shown only after the beginning of the cercarial emissions, while a normal, or even increased fecundity was established in the prepatent period of the infected snails.