Experimental infections of the mollusc Biomphalaria pfeifferi by Echinostoma caproni miracidia (Mi) were carried out in order to analyze the migration and development of mother sporocysts (MS) at 26 C. Miracidia penetrated different parts of the host's body, such as the mantle collar, the foot and head covering (including velum and tentacles), the mantle cavity, and the oral cavity. The ventricle and common aorta were the final sites of infection for the mother sporocysts after migration. The path of migration of the sporocyst was influenced by the point of MS penetration, but in all cases the MS reached the ventricle through the venous system. Developmental studies showed that newly hatched Mi contained 5-7 germinal cells (the primary germinal cells) and some undifferentiated cells. During sporocyst development, every primary germinal cell apparently gave rise to a redial embryo, whereas undifferentiated cells gave rise to both somatic and secondary generative cells. The eventual degeneration of the sporocyst seemed to cause the end of the development of this germinal material. The MS produced about 15 mother rediae (MR). Intramolluscan development of E. caproni MS consisted of 5 main periods: (1) resting, (2) migration, (3) growth, (4) reproduction, and (5) degeneration. A lower temperature of 21 C affected the duration of each stage. However, the path of sporocyst migration, the pattern of their growth, and the developmental steps of the germinal material were similar.