Parasitoids are the main component of biological control of Lepidoptera defoliators in forested areas, and the densities of host insects can affect the efficiency of these natural enemies. The aim of this work was to study the biology and parasitism of Palmistichus elaeisis Delvare & LaSalle (Hymenoptera: Eulophidae) at various densities of females on individual pupae of Thyrinteina arnobia Stoll (Lepidoptera: Geometridae), the major Lepidoptera pest in eucalyptus (Myrtaceae) plantations in Brazil. Thyrinteina arnobia pupae were exposed to P. elaeisis females at various parasitoidto-host ratios: (1:1, 3:1, 6:1, 9:1, 12:1, 15:1, 18:1, and 21:1), with 12 replications. The parasitism (81.8%) and emergence (100%) rates of P. elaeisis on T. arnobia pupae were highest with 15 ovipositing females per host. The life cycle (egg to adult) of P. elaeisis was shortest (19.8 ± 0.5 d) at a ratio of 21:1. The largest production of P. elaeisis per T. arnobia pupa was obtained at the ratios of 15:1 (298.4 ± 5.0) and 18:1 (287.4 ± 4.3). The female sex ratio of parasitoid offspring was highest (0.97 ± 0.01) with a parasitoid-to-host ratio of 12:1. The density of 15 P. elaeisis females per T. arnobia pupa was the most appropriate one for providing high parasitism (81.8%), emergence (100%), and progeny (298.4 ± 5.0) of this parasitoid.