Simple SummaryIt is increasingly recognized that gut microbiota plays crucial roles in host health and function. Various ecological and physiological factors influence the structure of the gut microbial community, resulting in, for example, the formation of enterotypes or the development of inflammatory disease by dysbiosis in humans. Social insects, such as bees, ants, and termites, are known to harbor unique but stable gut microbiota among individuals, which can be a good model to understand how gut microbial communities are shaped and stably maintained in host populations. This review summarizes current knowledge regarding structures of gut microbiota in social insects. Microbes colonizing those insect guts and differentially abundant among host castes are mainly featured.AbstractGut bacterial communities assist host animals with numerous functions such as food digestion, nutritional provision, or immunity. Some social mammals and insects are unique in that their gut microbial communities are stable among individuals. In this review, we focus on the gut bacterial communities of eusocial insects, including bees, ants, and termites, to provide an overview of their community structures and to gain insights into any general aspects of their structural basis.PseudomonadotaandBacillotaare prevalent bacterial phyla commonly detected in those three insect groups, but their compositions are distinct at lower taxonomic levels. Eusocial insects harbor unique gut bacterial communities that are shared within host species, while their stability varies depending on host physiology and ecology. Species with narrow dietary habits, such as eusocial bees, harbor highly stable and intraspecific microbial communities, while generalists, such as most ant species, exhibit relatively diverse community structures. Caste differences could influence the relative abundance of community members without significantly altering the taxonomic composition.