We investigated the ultrastructure and the crystallographic orientation of spicules from the calcareous sponge Paraleucilla magna (subclass Calcaronea) by transmission and scanning electron microscopy using two different methods of sample preparation: ultramicrotomy and focused ion beam (FIB). It was found that the unpaired actine from the spicules was oriented in the [211] zone axis. The plane that contains the unpaired actine and divides symmetrically the paired actines is the (-1 2 0). This plane is a mirror plane of the hexagonal lattice system. All the spicule types analyzed presented the same crystallographic orientation. Electron nanodiffraction maps from 4 x 4 μm regions prepared by FIB showed disorientation of less than 2° between diffraction patterns obtained from neighbor regions indicating the presence of a unique highly aligned calcite crystalline phase. Among the eight FIB sections obtained, four presented high density of pores. In one section perpendicular to the actine axis pores were observed only in the center of the spicule aligned in a circular pattern and surrounded by a faint circular contour with a larger radius. The presence of amorphous carbon representative of organic molecules detected by electron energy loss spectroscopy was neither correlated with porosity nor to specific lattice planes.