Oligonucleotide primers were designed to amplify a 490-basepair DNA fragment in the 5′ end of the pWb 12 repeated DNA sequence in Wuchereria bancrofti for specific amplification of W. bancrofti DNA by the polymerase chain reaction (PCR). A single microfilaria in 100 µl of blood or added to 1 ml of blood, a single third-stage larva in a pool of 20 uninfected mosquitoes, or 0.4 pg of W. bancrofti genomic DNA added to 100 µl of human blood or serum can be detected by this PCR method. The parasite DNA in human blood and hydrocele samples and in mosquitoes was isolated free of any PCR inhibitors using simple purification techniques. Detection of PCR products was carried out by agarose gel electrophoresis, followed by staining with ethidium bromide and visualization under ultraviolet illumination. The results indicate that the PCR method is species-specific, rapid, and more sensitive than that of DNA probes and routine microscopy.