A total of 6,226 fecal samples collected from 1980 to 1996 in the Australian states of Victoria, New South Wales, and Tasmania from individuals with gastroenteritis were tested for small round-structured viruses (SRSVs) and classical human caliciviruses (ClHuCVs) by electron microscopy. There were 223 samples positive for SRSVs, and nine positive for ClHuCVs. SRSVs were detected in individuals of all ages and were commonly associated with gastroenteritis outbreaks in nursing homes and hospitals. SRSVs were detected throughout the year, but were more common in the period from late winter to early summer in Australia (August to December). There were peaks of virus activity in the early 1980s and more recently in 1995 and 1996. Analyses by RT-PCR and sequencing of a segment of ORF1 encoding the putative RNA polymerase for SRSVs and ClHuCVs showed the presence of viruses belonging to several genogroups. Viruses of genogroup 1 (Norwalk/Southampton-like) and genogroup 3 (ClHuCVs) were relatively rare. Viruses of genogroup 2 (Snow Mountain-like) were common, and could be divided into two subgroups, one containing Toronto/Mexico-like viruses, the other Lordsdale/Camberwell-like viruses. The majority of viruses detected belonged to this latter subgroup.