Blastocystis hominis is a unicellular organism the pathogenic potential of which in humans remains unclear. It may be identified during a workup for gastrointestinal symptoms, usually in stool examined for ova and parasites. We describe a case in which B hominis was identified by cytologic examination in a patient with Crohn's disease who underwent colonoscopy and brushing of a transverse colon stricture. The morphologic features of this organism are described and contrasted with those of the uninucleate cyst form of Entamoeba histolytica.