The midgut cells of cephalobaenid pentastomids contain spherocrystals varying in size and appearance between the genera. Iron was detected in the vicinity of the crystals in all three genera, being considerably fainter in Reighardia sternae than in the other species. Ultrastructurally a distinct lamination of the spherocrystals was evident, being faint in R. sternae but clearly expressed in Raillietiella hemidactyli and Cephalobaena tetrapoda. According to the species, their diameter ranged from 1.3 to 6.25 microns. The size and number of the crystals were highest in R. hemidactyli and lowest in R. sternae. The inclusions were formed in the endoplasmic reticulum and migrated toward the cellular apex, accumulating there and being expelled into the midgut lumen. As determined by energy-dispersive X-ray analysis, calcium turned out to be the main component of the crystals. Furthermore, small amounts of chlorine and iron could be traced in the crystals. The relevance of the crystals is regarded as a kind of storage excretion such as that present in many other arthropods.