Investigations were carried out on the influence of the local environment on the preparasitic stages of B. phlebotomum and the following observations were made.

1. The infective larvae are positively thermo-tropic and positively phototropic. These observations confirm those of Schwartz (1924). The larvae are not negatively geotropic and do not climb grass, but remain in the dung, gaining access to the host by adherence of the dung to the skin.

2. Lack of air, such as would occur in tightly packed faeces; immersion of the faeces in water; temperatures below 10° C.; and lack of moisture, are all inhibitory to development. Infective larvae are resistant to at least 5 days‘ direct exposure to an atmosphere of relative humidity 75 at a temperature of 25° C.; in dry faeces they are resistant to 14 days‘ exposure to the same atmosphere.

3. In Northern Nigeria desiccation is the most important inhibitory factor in development. The extreme dryness of the pastures in the dry season prevents development of the larvae and adherence of dung to the skin. Penetration of the skin of the host probably occurs only in the rainy season, maximal penetration probably occurring just after the rains are regularly established.

4. The female lays 600 eggs in 12 hr. in the early rains, but the output of eggs by the females varies considerably throughout the year. It is greatest in the early rains and lowest in the later months of the dry season.

5. The female hookworm burden of nomadic cattle fluctuates throughout the year. The maximum burden occurs in the later months of the dry season.

6. Three factors, all probably bound up with a single factor, i.e. moisture, seem to influence the degree of pasture infectivity. (a) Inhibition of larval development; (b) failure of faeces to adhere to the skin; (c) fluctuation in the egg-laying rate of the female hookworms. The fluctuating pasture infectivity results in a fluctuating hookworm burden.

7. The period of maximum hookworm burden in nomadic cattle more or less coincides with the period of incidence of a disease in Stock Farm cattle, associated with hookworm infestation.

In the discussion an attempt is made to correlate these observations both in accounting for the seasonal incidence of ‘hookworm disease’ and in suggesting methods of control.

This paper was written during the tenure of a Cooper Centenary Fellowship for which the author is indebted to the Council of the Veterinary Educational Trust. The writer's thanks are also due to his laboratory assistant in Nigeria, Mr R. A. O. Shonekan.