This thin-section transmission electron micrograph (TEM) revealed some of the ultrastructural morphology found within the cyst-stage of a Giardia sp. protozoan. The outer cyst wall is composed of filamentous and membranous portions, and is separated from the cytoplasm of the trophozoites contained within by the peritrophic space. This cyst wall is approximately 0.25 microns thick. The protozoan Giardia causes the diarrheal disease called giardiasis. Giardia species exist as free-swimming (by