Cyst walls of Entamoeba invadens were isolated and purified. Both whole cysts and purified walls appeared intensely fluorescent when stained with Calcofluor white M2R. Examination of positive replicas of purified cyst walls with the electron microscope revealed the presence of a microfibrillar structure. The main sugars detected in acid hydrolysates from the walls were hexosamines. X-ray diffraction analysis of purified cyst walls demonstrated that the crystalline polymer present was chitin.