The results reported in this paper can be considered as a starting point for the molecular characterization of certain closely related Aphidiinae species belonging to the genus Aphidius Nees, of relevant interest in biological control. The species considered are: Aphidius ervi Haliday and A. microlophii Pennacchio & Tremblay belonging to the ervi group, Aphidius urticae Haliday and A. smithi Sharma & Subba Rao belonging to the urticae group. The present paper aimed at investigating the rDNA structure of these insect groups and evaluating the real potentialities of this molecoular tool, both in discrimination of species and determination of phyletic relationships between the taxa. What we considered as only a starting point for the study of the molecular genetics of the genus Aphidius nevertheless enabled us to discriminate the differences within and between four species of Aphidius and to conclude, with respect to the rDNA structure, that: a) A. smithi is the most different of the four species, b) A. smithi and A. urticae greatly differ from the two species of the ervi group, c) A. ervi and A. microlophii, although very similar, present significant differences, d) A. microlophii is less variable than A. ervi. This approach could provide a new interesting insight on the bio-evolutionary status of closely-related species and intraspecific populations and the resulting deeper biosystematic knowledge might increase the possibility of a successful utilization of these beneficials in biological control programs. none