Here we provide the first description of a natural infection of members of the beetle family Monotomidae with neogregarines and specifically the first finding of such a pathogen in the predatory beetle Rhizophagus grandis. The fat body of the beetle is the site of infection, and the typical navicular oocysts are 11.87 +/- 0.67 pm in length and 6.96 +/- 0.43 pm in width (n = 60). Polar plugs are recognisable using light and electron microscopy. The oocyst wall is quite thick, measuring 400 to 500 nm. Oocysts are formed pairwise within a gamontocyst, and each oocyst has eight sporozoites. The described neogregarine pathogen in R. grandis has the typical characteristics of members of the genus Mattesia (family Lipotrophidae) within the order Neogregarinida.