The effects of Bayluscide were studied on both immature and adults Bithynia siamensis goniomphalos for its molluscicidal efficiency. The lethal concentration (LC50) values of immature and adults were 0.28 and 0.17 mg/l, respectively and LC95 values were 0.60 and 0.49 mg/l, respectively. The effects on non-target aquatic animals, with no lethal effect to Hoi Kom (Filopaludina martensi martensi), slightly effects to guppy fish (Poecilia reticulata) but highly effects to Pla Sew Kaosan (Oryzias mekongensis). Histological studies of the effects of Bayluscide on B. siamensis goniomphalos were performed by paraffin sections and hematoxylin-eosin staining. Separation of epithelial layer of digestive tracts and muscle layers were clearly defined in the treated groups. Cells of digestive glands were swollen, shrinkage of calcium cells and their lumens become narrower while no changes in other tissues compared to the control group.