The green dock beetles of the genus Gastrophysa preferentially feed on broad-leaved dock Rumex obtusifolius, a relatively common weed plant and an invasive species originating from Europe, which causes ecological and economical problems in many countries. Oxalate, which is generally toxic to animals, is present at a high concentration in the leaves of R. obtusifolius but accumulates in the bodies of Gastrophysa atrocyanea only when treated with antibiotics. This suggests gut microbiota of G. atrocyanea may play a significant role in oxalate degradation and thereby enables Gastrophysa beetles to live on R. obtusifolius. To understand the gut microbiota in detail, we constructed and analyzed metagenomic 16S rRNA gene libraries of gut microbiota in G. atrocyanea and Gastrophysa viridula collected in multiple sites in Japan and Germany, respectively. The family Enterobacteriaceae dominated the gut microbiota of both Gastrophysa species, and phylotypes affiliated with the species Rahnella, Serratia, and Pantoea formed largest populations. Interestingly, 16S rRNA gene sequence of the phylotype of Rahnella was almost identical to that of a novel strain isolated from G. atrocyanea using semi-synthetic medium supplemented with oxalate. A gene homolog of oxalate decarboxylase, which sequence was assigned to Enterobacteriaceae, was found in the metagenomic library of G. viridula, which suggests that oxalate is converted to formate in the gut of the beetles.