Different morphological forms of Blastocystis hominis had been identified in human stool samples. These included both cystic and trophic stages. The latter was induced to encyst by keeping them in potassium dichromate solution for two weeks. Suspected of being the infective stage, cysts were studied in more detail as regards their morphology using both light and electron microscopy. Histochemistry and infectivity studies were also carried out. Light microscopy revealed the cysts to be ovoid or round 5-7 microm with thick cyst wall and a single nucleus. The induced cysts were morphologically similar to the cysts present in the stool. By electron microscope, the cyst wall was evident surrounded by an additional fibrillar coat. The cytoplasm contained one nucleus, many mitochondria, glycogen deposits and a number of variable sized vacuoles. Histochemical studies detected carbohydrates in the cyst wall and fat globules in the cytoplasm. Oral inoculation of albino mice with these cysts led to inflammatory changes in the large and small intestine. The parasite was found at the mucosal epithelium but with no invasion. Different forms of the parasite were detected in the lumen of the intestine.