Schistosomiasis is a public health problem in many developing countries. The mollusc Biomphalaria glabrata is the most important vector of Schistosoma mansoni in South America. The population control of this vector to prevent the spread of schistosomiasis is currently done with the application of highly toxic molluscicide to the environment. The screening of substances in sublethal concentrations that have deleterious effects on physiological parameters is very relevant for the control of schistosomiasis, since the effectiveness of disease prevention increases if it acts on population control of the vector and on reproduction and elimination in S. mansoni cercariae. The objective of this study was to evaluate the reproductive parameters (fecundity and fertility), intramollusk effect (sporocysts I (72 h) and II (14 days after)) on the development of cercariae of S. mansoni and the immune cell profile of B. glabrata exposed to sublethal concentrations (LC25 - 0.5 μg/mL and LC50 - 0.92 μg/mL) of the usnic acid potassium salt (potassium usnate). LC 25 and LC 50 significantly reduced (p < 0.05) the fecundity of B. glabrata when treated infected and/or not exposed to infection, while unviable embryos were not observed in sporocyst stage I, being only significant (p < 0.05) for mollusks infected and treated with LC50 on sporocyst II. LC25 and LC50 of the potassium usnate caused significant reductions (p < 0.05) in the production and cercarial shedding when evaluated on sporocysts I and II. In addition, the mortality of infected and treated B. glabrata in the sporocyst II phase was quite marked after the 9th week of infection. Regarding the immunological cell profile of uninfected B. glabrata, both concentrations led to immunomodulatory responses, with significant morphological changes predominant of hemocytes that entered programmed cell death (apoptosis). It was concluded that the application of LC25 and LC50 from the potassium usnate could be useful in the population control of B. glabrata, since it interferes both in their biology and physiology and in the reproduction of the infectious agent of schistosomiasis mansoni. Keywords: Biomphalaria glabrata; Molluscicidal; Fecundity and Fertility; Sporocysts I and II; Schistosoma mansoni; Hemocytes.