A recent report has described the molecular cloning and characterization of a novel, single-stranded DNA virus, named TT virus (TTV), which was present in the sera of Japanese patients with posttransfusion hepatitis of unknown etiology [Okamoto et al. (1998) Hepatology Research 10:1-16]. Using a nested polymerase chain reaction assay, sera from Brazilian patients with acute non A-C hepatitis and blood donors were examined for the presence of TTV DNA sequences. Thirty-seven of 52 (71%) patients with acute non A-C hepatitis and 45 of 72 (62%) blood donors were found to have TTV sequences in their sera. Such a high proportion in blood donors indicated that TTV infection is common in the general Brazilian population. Partial nucleotide sequences (326 bases in open reading frame 1) from seven isolates were determined. By phylogenetic analysis, four TTV strains were classified into the genomic subgroup G1a described previously. The three others belonged to subgroup G1b. Sequence homologies between strains belonging to a same subgroup were 92.9-99.1%, whereas homologies of 85.9-90.2% were calculated between isolates from different subgroups.