Specimens of the “living star-sand,” Baculogypsina sphaerulata, were collected from Lizard Island, Great Barrier Reef, Australia. Detailed cytological study of these algal-bearing foraminifera helped us to understand for the first time the relationship of the endosymbiotic diatoms to the complex morphology of the Baculogypsina test and its canal systems. We find that the symbionts are widely distributed in most of the chamberlets, especially in the umbilical and central parts of the test. In the proloculus and inner chamberlets, the symbionts are somewhat less abundant. Furthermore, the fossil canal liners and the expanded internal pore domes were empty of individual symbionts. It is now clear that regarding B. sphaerulata there is no special relationship between the symbionts and the homologs of the pores and liners as found in Amphistegina spp.