Giardia lamblia, originally considered a commensal enteric organism, is now universally noted for its pathogenic potential in man. The mechanism of pathogenesis of giardiasis, however, remains unclear. We report a case of giardiasis diagnosed by analysis of peritoneal fluid obtained from a trauma victim. G lamblia trophozoite forms were found on microscopic examination of the fluid, and chemical studies of the fluid revealed bile and elevated amylase activity. The trophozoites may have entered the peritoneum through bowel perforation.