The midgut of Raillietiella sp. was investigated in different ontogenetic stages. The developing embryo, the hatched first-stage larva, the infective fourth-stage larva from the intermediate host, and juveniles and adults from the definitive host were viewed ultrastructurally. From the fourth-stage larva onward, spherocrystals were visible in the midgut cells. They contained only small amounts of calcium and iron, depending on the ontogenetic stage. The cycle of crystal formation and destruction or excretion, respectively, is described herein. Their role in ion regulation and excretion is discussed with regard to similar inclusions in other arthropods.