It was found that in six aged horses after treatment with 30 grammes of phenothiazine the faecal egg count fell to zero but had returned to a high level after some live or six weeks. Examination of the faeces at the time of treatment showed that many worms were eliminated. Repetition of the treatment several times produced a similar result on each occasion, but there was a tendency for the egg count not to rise so high as it was before treatment, for fewer worms to be eliminated at each treatment, and for the time before the egg count reached a high level after treatment to lengthen with each successive treatment. Many treatments had to be given over a period of some three years before the egg counts of the horses were reduced permanently to a low level. Post-mortem examination of three of the horses at this stage showed their worm burdens to be low.

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The development and survival of the eggs and larvae of T. colubriformis was studied by spreading the daily faecal output of an infected sheep for 1 or 2 weeks on grass plots 6 ft square. The observations were carried out for 3 years and broadly similar results were obtained in each year. Eggs placed on the ground from November to February failed to develop. A large proportion of the eggs placed on the ground in March disintegrate but those which develop do so slowly. Infective larvae are not produced for 8–12 weeks and they are relatively short lived. As conditions become more favourable development becomes more rapid, taking 8 weeks in April 6 weeks in May and only 1 week in June, July and August. A larger proportion of the eggs develop into infective larvae and their time of survival increases. On most plots larvae did not die out until the following April. The prolonged period of snow during the winter of 1962–63 tended to preserve larvae rather than kill them. The relevance of these results to the control of trichostrongylosis is discussed and it is concluded that it would be impossible under British conditions to devise a system of rotational grazing which would satisfactorily control trichostrongylosis and at the same time ensure economic utilization of the herbage.We are grateful to Mr C. R. Phillips and Mr T. Doy for technical assistance during the course of this investigation.