It is possible to give a graphic representation of the effect of two pesticides applied jointly or of the effect of pesticides to which substances have been added. Each of the components is expressed in a coordinate. In the graph a quantitatively defined effect, e.g. a mortality of 50%, is inserted. These values are obtained by interpolation. The line which connects the points is called an isobole afterLoewe & Muischnek (1926) who used it for drugs. With an isobole the effect of different proportions of the components can be evaluated. The various types of isoboles are described and illustrated for some experiments with insecticides, herbicides and fungicides.