Phytases (EC 3.1.3) are widely used in animal feed to increase the availability of phosphorus and decrease the anti nutritive effect of myo-inositol hexakisphosphate (InsP₆). The aim of this work was to investigate the stereospecific degradation of InsP₆ in vitro and in vivo by a phytase from Citrobacter braakii (C. braakii), and to study gastric survival of the phytase as well as the site of action in the gastrointestinal tract. The in vitro results showed that the C. braakii phytase belongs to the group of 6-phytases (EC 188.8.131.52). However, in approximately one out of 10 instances the phytase initiated hydrolysis at the D-3 (L-1) position, demonstrating that phytase specificity is not unambiguous. Following the main degradation pathway, InsP₆ was degraded by stepwise removal of the phosphate groups on positions 6/1/5. The stereospecificity was found to be similar under in vitro and in vivo conditions. The phytase was found to be stable in the gastric environment and to be active in the stomach and possibly also in the proximal small intestine. While InsP₄ was accumulated under in vitro conditions this was not the case in vivo, where both InsP₅ and InsP₄ were seen to be hydrolysed in the small intestine, possibly as a combined action of the C. braakii phytase and endogenous phosphatases present in the mucosa. The ability of the C. braakii phytase to focus its activity on degrading InsP₆ to InsP₄ is believed to be a favourable complement to the endogenous phosphatases.