The oviposition behaviour of Oscinellafrit L. was studied in a series of greenhouse experiments. The crevice between the coleoptile and the plant base in oat seedlings was strongly preferred by the frit fly female as oviposition site. A simultaneous reception in the female of (a) a tactile stimulus from the coleoptile crevice or a soil crack and (b) a chemical stimulus from the plant appeared essential for a high oviposition rate. The oat plants were maximally susceptible to oviposition around the two-leaf stage, when the coleoptiles as a rule were disconnected from the plant bases but not yet wilted. Three selective advantages associated with oviposition behind the coleoptile of oat seedlings in the two-leaf stage may be pointed out: (1) Frit fly larvae hatching on oat plants in the two-leaf stage have achieved an optimal compromise between the risk of not being able to penetrate too old a host plant and that of starvation on too small a plant, (2) larvae hatching near the base of the plant have the shortest way to penetrate before they reach the meristematic region, where the feeding takes place, and (3) eggs placed behind the coleoptile are protected against predators.