In a small-scale trial in which boxes of grass infested with Trichostrongylus colubriformis were treated with methyl bromide, a dose of 100 mg. hr./l. prevented guinea-pigs picking up the infection when they grazed the grass at a later date. A practical method is described for fumigating small areas of pasture. In a field trial, sheep were used to test plots of pasture fumigated with doses which did not kill the grass but the results were inconclusive. These experiments are discussed and also the possible value and applications of this technique.

I am indebted to Professor B. G. Peters and to the other members of the Imperial College Field Station, particularly Dr A. B. Page and Dr O. F. Lubatti, for their help and criticism during the work. Thanks are also due to Dr William Davies, the Director of the Grassland Research Institute, Hurley, for permission to carry out the field experiment there, and also to Dr C. R. W. Spedding and Mr T. Brown for their co-operative assistance and advice throughout this work.