Eggs, larvae and pupae of the yponomeutid Plutella xylostella were sampled weekly in a cabbage field in Mie Prefecture, Honshu, Japan, in 1988 and reared in the laboratory to determine the mortality factors of each stage. The seasonal abundance of adults was investigated using a synthetic sex pheromone trap. Adult population densities increased from mid-April to early June and decreased from late June to late July. During the period of increase, the mortality rates for the immature stages caused by parasitoids were low; they were considerably higher during the period of decrease. The rates of parasitism by the larval parasitoid Apanteles plutellae [Cotesia plutellae] and the pupal parasitoid Diadromus collaris (both specific parasitoids of this host) were high from mid-June to mid-July, when the host was abundant. Tle rate of parasitism by the egg parasitoid Trichogramnza chilonis increased after late June, when the host was less abundant. A high rate of parasitism was also shown by Terrastichus sokolowskii, a larval-pupal parasitoid, after late July, when the host was less abundant. T. sokolowskii and Trichogramma chilonis are polyphagous parasitoids with very wide host ranges. none