Abstract Adult Onchocerca volvulus were isolated from nodules removed from onchocerciasis patients at four locations — two in the West African Sudan-savanna region (near Bamako, Mali, and Touboro, Cameroon), one in a West African forest region (Kumba, Cameroon) and one near Guatemala City, Guatemala. Four different cDNA expression libraries were constructed in bacteriophage λgt11 using poly(A) + RNA from the adult female worms. Individual cDNA clones of single copy genes were used to compare the genomes of parasites from the different locales and to show that the haploid genome of O. volvulus is 1.5 × 10 8 base pairs. About 1 in 700 recombinant clones in each of the four amplified cDNA libraries produces a fusion protein recognized by pooled human anti- O. volvulus antisera. Partial sequence determination of a 2.0 kb cDNA clone for an O. volvulus protein that induces an immunodominant response in rabbits revealed that this antigen has sequence similarities with Caenorhabditis elegans myosin and with schistosome paramyosin (which confers partial protection against schistosome infection). The four cDNA libraries have been deposited with American Type Culture Collection (ATCC), 12301 Parklawn Drive, Rockville, MD 20852, U.S.A., for general distribution under ATCC Number 37509.