Oviposition behavior of Zatypota albicoxa (Walker) is described. This wasp parasitizes the house spider Achaearanea tepidariorum (Koch), which weaves irregular, three-dimensional webs. Based on field observations, two modes of oviposition behavior were recognized. In one, the wasp hung on the web, pulling the thread with its fore leg, until the spider lifted it up (ambush-style). In the other mode, the wasp climbed the web (climbing-style). Under laboratory conditions in an aquarium, the wasp hung on the web in mid height of the gumfoot thread as in the former style, then flew toward unaware spider and paralyzed it. After paralyzing, the wasp usually rubbed the spider's abdomen with its ovipositor and tip of metasoma repeatedly at short intervals for several minutes. In all cases they adopted the same posture in which they grasped the spider abdomen with fore and mid legs during oviposition. As female wasps emerged from larger hosts and male wasps emerged from smaller ones, the ovipositing wasp apparently assesses the size of the spider and chooses whether to lay a fertilized or an unfertilized egg. In addition, it was confirmed that Z. albicoxa expelled the eggs not from the tip of the ovipositor but from the tip of its abdomen, as in other species of the Polysphincta group (e.g. Hymenoepimecis argyraphaga, Reclinervellus tuberculatus and Schizopyga circulator).