Four subtypes of hepatitis C virus (HCV), Pt(I), K1(II), K2a(III) and K2b(IV), have been suggested based on the nucleotide sequences of the non-structural (NS) 5 region. A fifth subtype from Japanese patients, Tr(V), which shows a less than 68% homology in nucleotide sequence when compared with other subtypes has been identified. A one-step method which enables a quick determination of subtype using polymerase chain reaction with a mixed primer set deduced from the sequence of each subtype has been developed. Using this technique, the subtypes of 418 out of 478 Japanese patients (87.4%) were determined. The incidence of each subtype in Japan was as follows: K1(II), 307 (73.4%); K2a(III), 74 (17.7%); K2b(IV), 28 (6.7%); and Tr(V), 3 (0.7%). This one-step subtyping technique should be useful for studying the epidemiology or biology of the HCV.