none Host transfer trials with six strains of the cereal aphid parasitoid Aphidius rhopalosiphi De Stefani-Perez revealed the presence of two differing host ranges: three strains parasitized Sitobion avenae (F.) and S. fragariae (Wlk.) exclusively, whereas the others successfully developed in both Sitobion species and also in Metopolophium dirhodum (Wlk.), M. festucae cerealium Stroyan, Rhopalosiphum padi (L.) and Diuraphis muehlei Borner. The two host range groups were poorly distinguishable by means of morphological characters. In contrast, isoelectric focusing proved to be a much better tool for discrimination. Reproductive isolation between the two host range groups was clear but not complete, as evidenced by crossing experiments with laboratory-bred and field-collected material. Under field conditions, males showed a specific response to virgin females of the two groups. Hence, the sex pheromones of the females were obviously different. Considering these results, a species (Aphidius sp. A) closely related to A. rhhopalosiphi seems to occur. As a detailed study of morphological charceris is still lacking, Aphidius sp. A is not yet formally established here. Due to its dependence upon Sitobion spp., Aphidius sp. A may be an economically important parasitoid of the pest aphid S. avenae.