European parasitoids considered for introduction to control the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), in North America have no previous association with the target pest. In Europe, they attack primarily Lygus rugulipennis Poppius and L. pratensis (L.). Compatibility of these parasitoids and their hosts in a “new-association,” the specificity of such biocontrol agents, and the potential competition with native parasitoids are important considerations for use of European species in biological control programs against L. lineolaris. In-host compatibility and competitiveness of the exotic multivoltine parasitoids Peristenus stygicus Loan and P. digoneutis Loan with the native parasitoids Leiophron lygivorus (Loan) (multivoltine), P. pallipes (Curtis) and P. pseudopallipes (Loan) (both univoltine) were assessed. Dissection of hosts indicated that >92% of the parasitoid attacks on L. lineolaris nymphs resulted in oviposition and development of the larvae for the five species studied. In suitability tests, 84% of P. stygicus adult wasps emerged from parasitized tarnished plant bugs, compared with 67 and 69% for P. digoneutis and L. lygivorus, respectively. No emergence was recorded for P. pallipes, and only 2.9% of adult P. pseudopallipes emerged. P. digoneutis was superior in the in-host competition with P. stygicus and L. lygivorus. Peristenus stygicus was dominant in the in-host competition with the three North American parasitoids. The size of the dominant larva inside the host for P. stygicus 7 d after oviposition decreased as the number of larvae increased.