Noroviruses are highly contagious, infecting both humans and animals. At least 10 genogroups exist (GI–GX), which are further divided into genotypes with the GII genotypes causing most outbreaks (1). The human norovirus capsid protruding (P) domain binds histo-blood group antigen (HBGA) co-factors for infection (2, 3), and a recently discovered bat norovirus (GX) was also found to interact with HBGAs (4). The genetic variability of the capsid gene is coupled with antigenic diversity and associated with escape from herd immunity, emergence of antigenic variants, and possible changes in host tropism (4–13). In this study, we determined the P domain structures of four human noroviruses and one bat norovirus using X-ray crystallography to examine structural features linked to potential interspecies transmission.