Field studies indicated thatApanteles yakutatensiswas a relatively common parasite ofAutographa californicaon peppermint. Adult parasites were active throughout much of the growing season (June through September) and were present at all locations sampled. Parasitism byA. yakutatensiswithin individual samples of field-collected 3rd- and 4th-instar hosts ranged from 7 to 71%. LarvalA. yakutatensisdevelop as gregarious endoparasites in 2nd- to 5th-instar hosts. An average of 29 parasites emerged per host with a highly variable sex ratio, generally 4:1. A hyperparasitoid,Stichtopithussp., was observed to be rare.