Symbiotic gut microbes such as bacteria enable host animals to digest specific diet items. Plant-eating animals like primates especially rely on symbiotic gut microbes for fiber digestion because plant fiber cannot be digested by host digestive enzymes. Recent next-generation sequencing studies have revealed relationships between host diet characteristics and microbiome abundance and function at the molecular level. I here show three microbiome studies in primates in terms of diet adaptation, that is, in chimpanzees (omnivorous), colobines (folivorous), and lorises (gumnivorous). The omnivore, chimpanzees showed similar pattern to another omnivore, hunan in microbiome composition. The microbiome analyses in colobines and lorises found that the folivore (leaf-eating) and gumnivore (gum-eating) have specific bacteria presumably related to digestion of leaf and gum fiber, respectively. This is because natural or artificial alteration of leaf or gum feeding in their diet repertoire could affect drastic change in abundance of specific bacteria. These studies provide an overview of adaptation of gut bacteria to the diet in plant-eating animals.