We have sequenced an HDV RNA from an acute delta hepatitis patient from the Nauru Islands. By comparison with other HDV sequences previously reported, we have identified three conserved regions: the first one is the sequence around the catalytic cleavage site for genomic-sense RNA; the second is the corresponding site for the antigenomic sense RNA; and the third is the sequence encoding the middle domain of the hepatitis delta antigen. We have shown that the middle domain of the delta antigen can bind specifically to the HDV RNA. This binding was demonstrated both in vitro and in the purified virion. It was suggested that this RNA-protein interaction is important for HDV RNA replication. We also suggest that the conserved sequences provide ideal primers for use in polymerase chain reaction (PCR) in clinical diagnosis of HDV infections. We recommend the use of nucleotides 870-900 and 690-720 (Makino et al., 1987a) as primers for routine screening.