Rhopalomastix is a bark digger ant that nests in the bark of living trees. The ants share a mutualistic relationship with diaspidids (armoured scale insects), housing them in their nest tunnels in the bark of a tree. Two colonies of Rhopalomastix sp. were sampled from two cultivars of mango trees in a mango plantation in Thailand. The architecture of the nest was mapped out and the respective resident diaspidid partners collected. Rhopalomastix built two different types of tunnels, one chewed through the live wood while the other was created by covering fissures in the bark with a frass roof. Tunnels were constructed at different depths and were inter-connected to form a complex network. Ant brood and diaspidids were not evenly distributed, with most found in deeper and wider chewed tunnels. A new species of pupillarial diaspidid, Ligaspis sp. was found in both colonies - this is the second report of a pupillarial diaspidid being associated with Rhopalomastix. We discuss the trophic relationship between ants and their diaspidid partners, and the potential impact this association has on host fruit trees.