none Two European parasitoid species were released in Ontario during the late 1970's to control the alfalfa blotch leafminer, Agromyza frontella (Rondani)(Diptera: Agromyzidae). One of these, Dacnusa dryas (Nixon)(Hymenoptera: Braconidae), rapidly became established and the other, Chrysocharis liriomyzae (=C. punctifacies) (Delucchi) (Hymenoptera: Eulophidae) was never recovered in Ontario. In 1999, we found both D. dryas and C. liriomyzae parasitizing first-generation A. frontella in Ontario in 1999. The combined parasitism rate for both species as revealed by larval dissections was 97.5% by the end of the first A. frontella generation. Of the adult parasitoids reared, 86% were D. dryas and 14% were C. liriomyzae. Most parasitized larvae contained a single unencapsulated (i.e., healthy) larva, along with one or more encapsulated eggs. No larvae were encapsulated, but the overall egg encapsulation rate was 47%. By the end of the first A. frontella generation, 86% of parasitized hosts contained at least one unencapsulated parasitoid and could therefore produce an adult parasitoid, and 12% of parasitized hosts escaped parasitism by containing only encapsulated parasitoids. The sex ratio of D. dryas was even at emergence, but strongly female-biased in sweep samples from the field. Egg loads of D. dryas females were all greater than zero and as high in the field as our highest laboratory estimates, suggesting that egg availability does not limit fitness under the conditions that we observed in the field. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land and freshwater zones; Nearctic region; North America; Canada Agromyza frontella (Agromyzidae); Hymenopteran parasites; Chrysocharis & Dacnusa; Records & parasitism rates; Ontario; Winchester; Hymenopteran parasite records & parasitism rates Dacnusa dryas (Braconidae ); Chrysocharis liriomyzae (Eulophidae ); Dipteran hosts; Agromyza frontella; Records & parasitism rates; Ontario; Winchester; Dipteran host records & parasitism rates