Summary. The cabbage root fly possesses highly specialised and extremely sensitive receptors for CIF, a group of compounds present on cabbage leaves in very small amounts, and the strongest oviposition stimulants known. Here we present evidence that the same receptors are sensitive to a methanol extract of cabbage root fly eggs, which may contain the flys host marking pheromone. Based on these results, the possible role of CIF in cabbage root fly behaviour and in cabbage plants is discussed.