Calcified worm fragments of adult Onchocerca volvulus from patients in West Africa were collected for a chemical analysis. The material contained predominantly calcium carbonate. Some results of investigations on the occurrence of calcified worms in untreated populations are reported on. Relations between the numbers of old parasites and those of calcified worms are discussed. The significance of possible resorption of calcified worm fragments for the interpretation of the effects of drug trials or other control measures are mentioned.