Aspidiotiphagus citrinus (Craw) is a solitary endoparasite of Unaspis yanonensis (Kuwana). Usually this parasite attacks larvae except the early first instar. There is a difference in the structure of scales between female and male larvae. For instance the first and second instar male larvae secrete coarse and fine cottony scales respectively, while females do not secrete cottony scales. To confirm the influence of the structure of scales on oviposition by the parasite, the author examined percentages of oviposition attempts, insertings with ovipositor and actual ovipositions to host-findings and the time required for ovipositional behaviour in female and male larvae of different developmental stages. These percentages and the time varied with the developmental stages. The percentage of actual ovipositions to host-findings in the second instar female larvae tended to be inverse relative to the time required for ovipositional behaviour. Such a relation suggests the possibility that thickness of scales is an important factor influencing oviposition. But the percentage of actual ovipositions to hostfindings in the early second instar female larvae was higher than in the late first instar female larvae in spite of the fact that there is no large difference in thickness of scales between them. This suggests hardness of scales is also an important factor influencing oviposition. Tapping and drilling with the ovipositor in male larvae were frequently disturbed by the secretion of cottony threads. Fine cottony scales secreted by the second instar male larvae had very large influence on oviposition. Thus, the early second instar female larvae were the most suitable for oviposition by the parasite.