Phytic acid is naturally occurring compound that reduces intestinal absorption of many metals. Early work suggests that some dietary phytate may be hydrolyzed in the large intestines by bacteria, but more recently nutritionists have suggested that a mucosal enzyme is responsible. This paper reports a study intended to resolve this controversy. The hydrolysis of dietary phytic acid was measured in germfree and conventional rats fed either of two diets that differed in their calcium content. Negligible phytate hydrolysis occurred in the germfree rats, whereas 22 and 56% of the phytic acid was hydrolyzed by conventional rats fed high- and low-calcium diets, respectively. We concluded that bacteria were responsible for the hydrolysis of phytate in these diets and that any activity of endogenous enzyme was negligible.