Summary.1The loss of seed potatoes through blight (Phytophthora infestans) in Jersey may be subdivided into (a) loss in the field at digging time, and (b) loss in the seed boxes subsequent to harvesting.2It is shown that the loss in the field may be prevented in most seasons by regular and thorough spraying, whilst that in the boxes may be reduced by scorching or removing the diseased haulms before digging, or by immersing the tubers in a fungicide.