An electron microscopic study of Crohn's disease of the colon is presented. The positive findings that supplement those obtained by light microscopy are: a prominent nucleolus is more common in the lymphocytes of Crohn's disease than in normal lymphocytes or those found in cases of ulcerative colitis; lymphocytes are often observed close to macrophages and epithelioid cells; the epithelioid cells are metabolically active, and have vesicles which contain acid phosphatase but little visible debris; intramural bacteria were identified in six of the 11 specimens of colon with intact epithelia and minimal inflammatory changes. The possible significance of these findings for the pathogenesis of the disease is discussed.