1. O. gutturosa infections are of widespread occurrence in South-East England. 58% of 1583 cervical ligaments from milk cattle slaughtered in an abattoir at Reading, Berkshire, showed the presence of adult worms. A more detailed study of 50 cattle from a small abattoir near St. Albans in Hertfordshire showed 42 (84%) infected.2. The adult worms were found either in the connective tissue adjoining the nuchal ligament or between the spleen and rumen. There was no evidence of an increase in the parasite density with age of the cattle suggesting a strong immunity to superinfection.3. Irrespective of the situation of the adult worms the microfilariae of O. gutturosa appear to have a directional mechanism and were always concentrated in the skin around the umbilicus. This is an adaptation to transmission by Simulium ornatum which bites preferentially in the umbilical region.4. There were no marked pathological changes associated with the parasites and no evidence of clinical disease attributable to infection with O. gutturosa.

The morphologies of the adult worms and microfilariae ofOchocercafound in the neck and gastro-splenic region of cattle in southeast England were compared. It is concluded that they are forms of the same species.

1. Observations were made on various methods of infecting S.ornatumwith 0.gutturosa, the behaviour of the parasite in its vector, and the maintenance of the vector in the laboratory.2. S.ornatumreared from pupae brought into the laboratory were successfully infected with 0.gutturosaafter feeding on infected cattle. The use ofin vitromethods of infection was unsuccessful as microfilariae were ingested but did not undergo larval development.3. The migratory route of 0.gutturosamicrofilariae ingested by S.ornatumwas studied and it was found that a mean of only 25% penetrate out of the abdominal midgut. The dimensional changes of 0.gutturosawere noted from the time of ingestion by S.ornatumuntil the infective stage was reached.4. 0.gutturosatook 13–15 days to reach the infective stage in S.ornatumwhen maintained at a constant temperature of 23°C.5. The development and structure of the peritrophic membrane of S.ornatumare described.6. Preliminary observations on the seasonal variations of some factors affecting the transmission of O.gutturosaby S.ornatumsuggest that microfilariae were only available for ingestion during the period of vector activity, after which they migrated into the deeper regions of the skin of cattle.