As a part of exploration of novel bacterial genes coding for insecticidal proteins, we have initiated structural and functional studies on oxalate decarboxylase from Photorhabdus luminescens, a symbiotic bacteria inhabiting guts of nematodes that parasitize insects. Oxalate decarboxylases (OXDC) are enzymes that catalyze the conversion of oxalate to formate and carbondioxide (CO2) in the presence of manganese and dioxygen. Structures of OXDC in complex with ethylene glycol (EDO), EDO/formate (FMT) and FMT/CO2 were determined at resolutions of 1.97Å, 2.36Å and 2.5Å, respectively. The enzyme is a trimeric molecule and each protomer consists of two cupin domains. All the three ligand bound structures of OXDC resemble the closed form of OXDC from B. subtilis reported earlier. Comparison of these structures with the open and closed forms of B. subtilis OXDC indicated Glu166 in cupin domain I but not in domain II is in a conformation suitable to function as the catalytic base/acid and hence only domain I may be catalytically active. It is observed that the hydrogen bonding interaction between Arg95 and Thr169 of cupin domain I is essential for the positioning of Glu166 for catalysis, which is absent in cupin domain II. An analysis of the similarities and differences between OXDC structures from P. luminescens and other reported bacterial OXDC structures and oxalate oxidase from Hordeum vulgare has been carried out with the view of understanding substrate and functional specificities of these enzymes and to investigate the mode of action of the enzyme, for developing P. luminescens as a suitable bio-insecticide against plant pests.