Several species of phytophagous Pentatomidae (Nezara viridula, Acrostern-um hilare, and Euchistis servus) have the potential to become major pests on transgenic B.t. cotton. These species were observed in transgenic B.t. cotton in South Carolina during 1995. Stink bug damage to maturing bolls occurred in mid to late season as adults migrated in from alternate hosts. Stink bug damage to bolls ranged from 15-71% in untreated B.t. cotton and 7-53% in B.t. cotton treated with methyl parathion. At one site, there were no significant differences in seed cotton yields from B.t. cotton that received one, two, or three applications of methyl parathion, whereas untreated B.t. cotton yielded significantly less than treated B.t. cotton. At all sites, untreated B.t. cotton produced numerically lower seed cotton yields than treated B.t. cotton. Seed cotton reductions in untreated B.t. cotton resulted primarily from stink bug damage to bolls. There were no significant differences between seed cotton yields from non-B.t. cotton treated with pyrethroid for bollworm and B.t. cotton treated with methyl parathion for stink bugs. Transgenic B.t. cotton treated with methyl parathion at South Carolina's arbitrary treatment threshold of one stink bug per six feet of row provided adequate protection during these tests. Economic thresholds for stink bugs should be established in B.t. cotton to protect the crop from potentially serious losses.