Development of test ultrastructure through ontogeny is traced in four living species from the foraminiferal families Bolivinitidae and Caucasinidae. Test construction conforms in general to the monolomellar concept, but differs in having double pore membranes between consecutive calcitic lamellae and in maintaining a nearly consistent wall thickness through out ontogeny. The test wall observed by scanning electron microscopy consists of an inner, spongy organic lining, a basal membrane, the calcitic wall with a surface veneer, and an outer membrane. Calcitic two-layered lamellae enclosed by organic membranes are added to the test with each new chamber. Single or multiple organic membrane complexes seal completely the majority of the pores and preclude the free exchange of protoplasm to the test surface.