The filarial worm Parafilaria bovicola causes streaks of blood on the skins of live cattle and slimy bruise-like lesions on the subcutaneous surfaces of their carcases. To determine the vectors of this worm a field survey was conducted in November 1972 and during the summer of 1973/74 on 6 farms in the Transvaal. A total of 10 093 flies was collected off cattle and examined for the infective 3rd stage Parafilaria larvae. Of the 12 fly species collected, Muscalusoria, Musca domesrica, Musca xanthomelas, Musca n. sp., Musca sorbens and Musca fasciata were the commonest. Third stage larvae were found in 3 species all belonging to the subgenus Eumusca, viz. M. fusaria, M. xanthomelas and a new Musca species. The infection rate in these flies was usually less than 1% and most of the worms were recovered from the heads of female flies. The same 3 species were successfully infected artificially in the laboratory, the infection rate ranging from 40, 0%-53, 8%. The measurements of larvae from these flies agreed closely with those of larvae recovered from field-collected flies. Female P. bovicola worms perforate the skin of cattle and deposit their eggs and/or microfilariae into the blood which trickles down from the holes. Most of these bleeding marks were noticed between July and December 1973. As 3rd stage larvae were recovered from flies from August 1973 to February 1974 the main period for transmission is likely to be between July and February. Since the 3 flies suspected of being vectors feed mainly on eye secretions it is believed that most transmission takes place via the orbital route. Thereafter development of the worms in cattle to the adult stage will take approximately 7-10 months.