Sporozoites of the coccidian, Eimeria tenella, identified in histological sections by their cytoplasmic glycogen, revealed a mode of penetration different from that previously assumed. Sporozoites, studied in sections of Cecal tissue obtained 12 and 24 hours after inoculation with 20-30 million oocysts, invaded the surface epithelium of the ceca at the tips of villi and proceeded across the basement membrane into the tunica propria, through which they passed, free or within cells which were probably macrophages. Sporozoites then invaded the epithelial cells lining the gland fundi. Noticeable damage in the form of passageways or "penetration tubes" in the striated border and epithelium was produced by the invasion process. The basement membrane at the villar tips acted for a while as a barrier against further invasion.