The tarnished plant bug, 'Lygus lineolaris' (Palisot de Beauvois) is an important native agricultural pest in Canada. Braconid parasitoids of the genus 'Peristenus' introduced from Europe could help to reduce 'Lygus' populations. Native parasitoids of mirids and their potential interactions with the introduced parasitoids were investigated before planned releases. Peak densities of nymphs of 'L. lineolaris ' and 'Adelphocoris lineolatus' (Goeze) were observed in mid-June and at the end of July/early August. Annual rates of parasitism (May-Sept.) varied between 3.7% and 7.7% for 'L. lineolaris' nymphs and adults, and 'A. lineolatus' nymphs, and varied with the time of sampling. Parasitism rates were much higher for another mirid, ' Leptopterna dolabrata' (Linnaeus). Peaks of parasitism corresponded to peak presence of 'L. lineolaris' and 'A. lineolatus ' nymphs. Parasitism rates on alfalfa were, in general, lower than those found on other host-plants. Five species of native parasitic Hymenoptera from 'L. lineolaris' have been collected in the Guelph area: ' Peristenus pallipes' (Curtis), 'Peristenus pseudopallipes' (Loan), 'Leiophron lygivorus' (Loan), 'Leiophron solidaginis ' Loan, and 'Leiophron' sp. near 'brevipetiolatus ' Loan. Laboratory experiments determined a threshold of development of 9.6°C for 'L. lineolaris', and emergence thresholds for the European parasitoid 'P. digoneutis' Loan and 'P. stygicus' Loan. Accumulated degree-days in the field above the thresholds, and predicted occurrence of parasitoids and various stages of 'L. lineolaris' indicated a delay of almost 30 days between initial parasite emergence and the appearance of 'L. lineolaris' nymphs. Predictions of ' L. lineolaris' appearance from laboratory studies do not seem to reflect peak collections of nymphs in the field. In-host compatibility and competitiveness of the exotic multivoltine parasitoids, 'P. stygicus' and 'P. digoneutis', and the native parasitoids, 'L. lygivorus, P. pallipes' and ' P. pseudopallipes', showed that >92% of the parasitoid attacks on plant bug nymphs resulted in oviposition and development of larvae. ' P. digoneutis' and 'P. stygicus' appear to be superior in-host competitors compared with the three North American parasitoids. In the laboratory, most 'P. stygicus' were induced into diapause when transferred to short day photoperiod prior to 6.5 days after oviposition at 22°C. Only a slight increase in diapause induction was observed for ' L. lygivorus' when reared at short day photoperiod. Both ' P. digoneutis' and 'P. stygicus' show potential for biological control of 'Lygus' bugs in Canada.