In most parts of the world helminth infestations are responsible for such a heavy wastage of sheep that their control has undoubtedly become one of the most important charges that devolve upon the flock-owner. It might be thought that in tropical regions suited to the raising of sheep the problem of helminth control would cease to be urgent; and that climatic conditions, the general aridity of land chosen in such countries for sheep-rearing, and its consequently smaller carrying capacity, would contribute to form an environment distinctly unfavourable both to the survival of the free-living stages of parasitic worms and to their chances of reaching a suitable intermediary or definitive host. Such areas are not usually well watered, and it certainly does result from the paucity of permanent streams and standing water that few species of trematodes are represented. Those species that do occur, e.g. Fasciola hepatica, F. gigantica, are restricted to particular localities and are easily subjected to control.