The effect of ovipositional experience on host discrimination by Trichogramma chilonis females was studied using Plutella xylostella eggs as hosts. Females of T. chilonis which had never oviposited in any host eggs after emergence was supposed not to distinguish between parasitized and unparasitized host eggs. However, experienced females distinctly discriminated parasitized host eggs from the unparasitized ones. Acceptability of unparasitized host eggs was 90% or more irrespective of host type which females had experienced. On the contrary, acceptance of parasitized host eggs decreased to a lower level; 15% in females which had experienced once with an unparasitized host egg and 60% in females which had experienced once with a parasitized host egg. Females experienced oviposition on an unparasitized host egg became to accept parasitized host eggs more frequently 24 after the experience.