The populations size of Biomphalaria glabrata and of B. tenagophila were studied comparatively during a six months period. A significantly larger initial development was observed for B. glabrata. Upon reaching a criticai saturation levei the number of specimens in the populations decreased. Following a short period of oscillation, the frequency of individuais attained a certain stability. A decrease in the frequency of oviposition preceded a lowering of the number of specimens observed after the saturation levei. This study suggests that limitating factors in the population development would have acted as a feed-back mechanism.