Isolates ofMetarhizium anisopliae(CNPSo-Ma12) andBeauveria bassiana(CNPSo-Bb56) were tested under field conditions as biological control agents of soybean stink bugs (Nezara viridula, Piezodorus guildinii,andEuschistus heros). Kaolin-based powder formulations ofM. anisopliaeorB. bassianawere applied to soybean plots at a rate of 1.5 × 1013conidia per ha. After treatment, field cages (0.25 m2) were placed in plots and stink bug adults were introduced into the cages. Mycosis for bothP. guildiniiandN. viridulawas initially observed 7 days postapplication in the 1991 season and at 15 days postapplication in the 1992 season. ForE. heros,mortality was detected on day 8 and on day 20 in 1991 and 1992, respectively. In 1991, infection levels of 48 and 41% were achieved at day 30 forP. guildiniiandN. viridula,respectively, whereas the infection level inE. herosreached only 33%. In 1992, mortality caused byM. anisopliaeon the three stink bug species was lower than that observed in 1991, with cumulative mycosis at day 30 being 15, 17, and 20% forP. guildinii, N. viridula,andE. heros,respectively. The 1991 season was humid and warm, whereas in the 1992 season there was an 8-day drought spell (<75% relative humidity (RH)) after application, suggesting that %RH was responsible for an earlier occurrence and a higher prevalence of both fungi on stink bug species in the 1991 trial. Despite the higher deposition ofB. bassianaon the plants (17 colony forming units [CFU] per mm2of leaflet) 1 day after application compared toM. anisopliae(5.1 CFU per mm2of leaflet),B. bassianawas less efficient against stink bugs thanM. anisopliae.Field results were confirmed through laboratory bioassays, in controlled conditions (T= 26± 1.5°C, 90% RH, and 8D:16L light regimen). The mean time to mortality byM. anisopliaewas 4.3 ± 0.2 days forP. guildinii,4.6 ± 0.2 days forN. viridula,and 7.4 ± 0.5 days forE. heros. E. heroswas less susceptible andP. guildiniiwas the most susceptible species toM. anisopliae.Since these species usually occur as a complex in soybean in Brazil, these results represent important information toward the development of entomopathogenic fungi as microbial insecticides of these pests.