Comparative studies were made on three presumed sibling species of the genus Eubazus, parasitoids of European Pissodes spp. weevils, to clarify their taxonomy and define diagnostic characters. Several populations of E. semirugosus (Nees), E. robustus (Ratzeburg) and Eubazus sp. were compared with respect to their morphology (mainly through morphometric analyses), fecundity, isoenzyme patterns and host preference. Crosses were made to assess the genetic and behavioural compatibility of the populations. In addition, the North American E. crassigaster (Provancher), a parasitoid of Pissodes strobi (Peck), was compared to E. semirugosus, a species selected for introduction against P. strobi in Canada. The ratio of the length of the ovipositor sheath to the fore wing length was the most discriminating morphometric variable, but discriminant analyses including several measurements were needed to completely separate European species. A canonical discriminant function provided a total separation between males of E. crassigaster and E. semirugosus, but not between females. Eubazus crassigaster and E. semirugosus were totally separated by the banding pattern of the enzyme phosphogluconate dehydrogenase whereas hexokinase and esterase provided a diagnostic separation between Eubazus sp. and E. robustus. Eubazus sp. differed from all the other species by having a greater number of ovarioles and, consequently, a higher potential fecundity. In a two-choice oviposition test, E. semirugosus and Eubazus sp. showed a significant preference for their natural host, P. castaneus De Geer and P. piceae (Illiger), respectively. A similar test made with their progenies reared under standard conditions showed that the difference in host preference was genetically fixed. Males and females of different species did not mate readily, in contrast to individuals from the same species. All attempts to interbreed E. robustus and Eubazus sp. failed, but a few crosses between E. semirugosus and the two other European species produced fertile offspring. These observations strongly suggest that the complex of Eubazus spp. parasitoids attacking Pissodes spp. in Europe is composed of at least three sibling species, two of which appear to have specialized on distinct host species that occupy exclusive microhabitats.