LC₅₀s of Dipel 2X, an endotoxin of Bacillus thuringiensis subsp. kurstaki, (HD-1 isolate) strain and Design Novartis CGA 237218, a transconjugated endotoxin B. thuringiensis subsps. kurstaki/aizawai hybrid strain were significantly different after 168 h. Strains were 15 and 28 fold more toxic against larvae of reference strains of tobacco budworm, Heliothis virescens (F.), than bollworm, Heliocoverpa zea (Boddie), respectively. Both strains had lower LC₅₀ values against bollworm and tobacco budworm than exotoxin thuringiensin (Di Beta}. Mixture of Dipel 2X endotoxin-exotoxin did not exhibit synergism against either species. At 168 h slope values of gut-disrupting endotoxins and protein inhibitor exotoxin for the bollworm and tobacco budworm ranged from 0.18 to 0.84 and 0.40 to 0.91, respectively. Slopes were flat, indicating that actions were probably multiple and took place at different times during the 168 h. No significant difference in LD₅₀, as measured by µg Dipel/larva by oral treatment of the tobacco budworm was shown for a field collected strain and the same laboratory reference strain. In a field test 58% to 85% control of third to fifth stage tobacco budworm larvae was obtained 2 to 4 d post-treatment after 2 spray applications of Design at 2, 2.5 and 3 kg/ha to first-second stage larvae. Six days post-treatment 39% to 64% control was determined for third-fifth stage of larvae at the same rates. No trend was shown for rates. Larval populations in untreated check plots exceeded the economic threshold of 0.05 larvae/terminal on all sample dates.