Almost all phytophagous stinkbugs (Heteroptera) possess symbiotic bacteria in their midgut ceca. Here we report the mechanism for transmission and acquisition of the symbiont in the Japanese bean bug Riptortus clavatus (Alydidae). The results of diagnostic PCR and rearing experiments rejected the possibility of vertical transmission. In contrast, almost all nymphs reared on soil-grown soybean plants acquired the symbiont, and only nymphs placed in the sterile bottles inoculated with the cultured symbiont acquired the symbiont. Body size and dry weight of the infected adults was significantly larger than that of the uninfected ones. From these results, we conclude that the bean bug acquires the benevolent symbiont not through vertical transmission but from the surrounding environment.