A field method of assessing the impact of ants as predators of forets insect pests, using a special apparatus, is described. A natural nest in the field is surrounded by an artificial barrier; the ants are made to enter it through a tube which permits only oneway movement. This tube passes through a sampling machine designed to collect the ants, together with prey they are carrying, at a precise fraction of each hour during the whole day. The machine can also be used in the laboratory with artificial nests. © 1969 Masson & Cie.