Abstract: Histological studies on the Schistosoma mansoni and Schistosoma margreboweie development in the lung of definitive host mouse have been reported after 2 to 21 days post-infection (dpi). During the lung phase of development shape of schistosomula were relatively thin, elongated and round. Histologically, the schistosomulum was characterized by an outer cuticle layer. The layer was variable in appearance because of very thin layers of underlying smooth muscle fibers and a homogenous layer with occasional ridge areas. The thin and wide regions of the body of schistosomulum was characterized by undifferentiated and differentiated cells with oval or irregular shaped nuclei rich in chromatin and little cytoplasm. Other structures included development of gut caeca with presence of natural or black pigment. Moderate type of infiltration of cells including a large numbers of eosinophils, some macrophages and a few lymphocytes were seen around S. margrebowiei but no such reaction was observed around S. mansoni schistosomula after 8 dpi. An appearance of plasma cell was also noticed close to the surface of S. mansoni schistosomula.