This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. Giardia lamblia, an intestinal flagellate, is generally thought to produce diarrhea in children and occasionally in adults. It has never been cultivated. The spread of the infection is through contamination with fecal material containing the cysts. It inhabits the upper part of the intestinal tract in the trophozoite stage. The fresh unstained stool specimens show numerous cysts and, in this case, an occasional actively motile trophozoite. The trophozoite is said to multiply by longitudinal fission. Detail of the flagellate is visible with ironhematoxylin preparations. The cysts are ovoid, and in iodine-stained smears nuclei, axostyles and fibrils may be seen. REPORT OF A CASE J. S., a boy aged 16, had had attacks of urticaria since the age of 9. He had lived alternately in Florida and Chicago for the past twelve years prior to examination. He had returned to Chicago a year previously. He was first seen in the Central