Calosoma sycophanta L. (Coleoptera: Carabidae) is a voracious predatory beetle that feeds on several important lepidopteran pests. It is mass reared in laboratory breeding and released against forest pests. Any infection in this beetle is undesirable because it impacts production of effective and healthy beetles. In the present study, a microsporidian pathogen was found in C. sycophanta. Mature spores of the pathogen are single, uninucleate, oval, and small, measuring about 3.04 x 1.7 mu m. The spore wall is 100 to 110 nm thick and consists of two layers: an electron-dense outer layer, the exospore (30-35 nm), and an electron-lucent inner layer (the endospore; 70-80 nm). The polaroplast is of the lamellated type with thin lamellae anteriorly and thick lamellae posteriorly. A polar sac with an anchoring disc of the polar filament is clearly seen anteriorly. The polar filament is isofilar and has 8 coils, each 70 to 95 nm in diameter. The microsporidium presented here is the first pathogen recorded from this beetle and is tentatively classified as Microsporidium sp.