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Pantoea agglomerans is an important bacterial symbiont of insects. Despite its frequent discovery in various insects, little is known of its strain diversity and quantitative dynamics. In this study, we investigated these aspects of the bacterium in the rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), a serious pest of rice in North America and East Asia. Based on the cloning and sequencing of a bacterial 16S rRNA gene, we identified three novel P. agglomerans strains from different weevil populations, with each population harboring only one strain. In female adults, gut was the dominant tissue harboring P. agglomerans. This bacterium was also present in the ovary, but its density was very low there. The bacterium was maintained at a low density across all immature stages (from egg to pupa); however, the density increased substantially as adults emerged. Its density was low in the overwintering stage, but increased stepwise after the overwintered adults were allowed to feed on host plants, i.e., when they developed from early to actively reproducing stages. The infection density returned to a low level in the adults at a late reproductive stage. In conclusion, each of the L. oryzophilus populations we investigated was probably infected by a single P. agglomerans strain, and the gut of adults is the major tissue/developmental stage to harbor this bacterium, with actively reproducing adults attaining a high level of infection.