The tetrameric short tandem repeat (STR) locus (D7S809) has been evaluated in the Japanese population. In order to detect the alleles, PCR was carried out using primers, one of which was end labelled with 32P, and PCR products were separated by electrophoresis on a denaturing polyacrylamide gel. Using this method, accurate genotypes could be determined from as little as 0.5 ng of genomic DNA. Thirteen different alleles were identified on 256 chromosomes tested. All alleles differed in size by one (4 bp) repeat unit, and no "interalleles' were found. The estimated heterozygosity and the polymorphism information content (PIC) were 0.86 and 0.83, respectively. We observed 42 of the 91 possible different genotypes. The power of discrimination (PD) was 0.96, and no significant deviations from the Hardy-Weinberg equilibrium were found. We retyped all apparently homozygous samples using an alternative pair of flanking primers in order to confirm homozygosity. We also demonstrated a typing result involving sexual assault. D7S809 appears to be a very useful STR locus for forensic practice in Japanese.