Astroviruses and caliciviruses may both be observed in the feces of babies by electron microscopy and are two of the viruses whose relationship to infantile diarrhea is still being established. Although similar in size and in their star-shaped surface configurations, they differ in detailed structure. The astrovirus has a round unbroken edge, a six-pointed star with a white center, and surface hollows that are triangular. The calicivirus has a feathery edge, a six-pointed star with a dark hollow in the center (Star-of-David) appearance), and surface hollows that appear round or oval. Because only a minority of the virions of both viruses exhibit a completely characteristic structure, careful searching may be necessary to distinguish between the two viruses.