The infective larvae of Haemonchus contortus (L3) obtained from an artificially infected sheep injected with 125 microCi of 59Fe, incorporated the isotope and retained it in sufficient quantities to be detectable by autoradiography in the adult worms which developed. This is apparently the first report of incorporation of 59Fe into adult helminths, of which the L3 originated from an animal that was injected with the isotope.