AbstractComparative studies were made on three presumed sibling species of the genusEubazus, parasitoids of EuropeanPissodesspp. weevils, to clarify their taxonomy and define diagnostic characters. Several populations ofE. semirugosus(Nees),E. robustus(Ratzeburg) andEubazussp. 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 AmericanE. crassigaster(Provancher), a parasitoid ofPissodes strobi(Peck), was compared toE. semirugosus, a species selected for introduction againstP. strobiin 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 ofE. crassigasterandE. semirugosus, but not between females.Eubazus crassigasterandE. semirugosuswere totally separated by the banding pattern of the enzyme phosphogluconate dehydrogenase whereas hexokinase and esterase provided a diagnostic separation betweenEubazussp. andE. robustus. Eubazussp. 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. semirugosusandEubazussp. showed a significant preference for their natural host,P. castaneusDe Geer andP. 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 interbreedE. robustusandEubazussp. failed, but a few crosses betweenE. semirugosusand the two other European species produced fertile offspring. These observations strongly suggest that the complex ofEubazusspp. parasitoids attackingPissodesspp. 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.