A series of field experiments was made on maincrop potatoes grown for seed with farmyard manure. The optimum rates of application of ammonium sulphate, superphosphate and potassium chloride were considerably lower than those at present practised in the area. Use of 120 lb. of N per acre, consistently depressed yield of both ware and seed. Moderate applications of superphosphate (60 lb. of P2O5 per acre) increased seed yield generally but had little effect with soils high in easily soluble phosphorus. Potassium chloride increased the ware/seed ratio considerably. Interactions between the effects of the fertilisers on yield were very small.