Field experiments on about 4700 sheep at about forty different farms in North Wales are discussed. Crutching is the most effective control measure for lambs. Regular dipping with commercial arsenic-sulphur dips gives reasonable control except during adverse weather conditions. Dipping with zinc arsenite or with calomel proved, on the whole, less effective than dipping with commercial dip. Addition of calomel to commercial dip proved to be more effective than commercial dip alone. Preliminary observations are presented on the variation of the surface tension and viscosity of the dipping fluid as the number of sheep passing through the dip increased.