A genomic library constructed in a bacteriophage lambda replacement vector (EMBL3) with Wuchereria bancrofti DNA partially digested with Sau 3A I was screened with 32P-labeled W. bancrofti total DNA, and a strongly reactive recombinant, EMBL3Wb34, was isolated. This clone contained an approximately 16-kb insert that showed some cross-hybridization with Brugia malayi and B. pahangi DNA. However, a 969-bp subclone from EMBL3Wb34, designated pWb12, hybridized only with W. bancrofti DNA and was able to detect as little as 300 pg. Furthermore, pWb12 could detect DNA from a single infective larva or one microfilaria. It has a moderate copy number (450-700) and appears to be interspersed within the parasite genome. The nucleotide sequence contains 66% A+T and 34% G+C and shows no notable internal repeats.