A bioassay method was developed to determine field deposits and persistence ofBacillus thuringiensisvar.kurstaki(B.t.k.) protein (in ng/g foliage, and ng/cm of foliar area), after aerial application of Foray®48B formulation over a hardwood forest. Two blocks (B1 and B2) were sprayed at 50 BIU in 3.94 L/ha. initial deposits and persistence of protein were assessed by two force‐feeding bioassay methods, foliar‐extract‐bioassay (FE‐bioassay) and redissolved‐protein‐bloassay (RP‐bioassay), using fourth instar gypsy moth (Lymantria disparL.) larvae.