The consumption of pomegranate products leads to a significant accumulation of ellagitannins in the large intestines, where they interact with complex gut microflora. This study investigated the effect of pomegranate tannin constituents on the growth of various species of human gut bacteria. Our results showed that pomegranate byproducts and punicalagins inhibited the growth of pathogenic clostridia and Staphyloccocus aureus. Probiotic lactobacilli and bifidobacteria were generally not affected by ellagitannins, while relatively small growth inhibition by ellagic acid likely resulted from decreasing media quality due to the formation of tannin-protein complexes. The effect of pomegranate ellagitannins on bifidobacteria was species- and tannin-dependent. The growth of Bifidobacterium animalis ssp. lactis was slightly inhibited by punicalagins, punicalins, and ellagic acid. POMx supplementation significantly enhanced the growth of Bifidobacterium breve and Bifidobacterium infantis.