The response ofEucelatoriasp. females in olfactometers to test materials was: okra leaves (1.00)>cotton leaves (0.52)>Heliothis virescens(F.) larvae fed okra leaves (0.32)>H. Virescenslarvae fed artificial diet and no okra leaves (0.0) =Spodoptera eridania(Cramer) fed cotton leaves (0.0) =Estigmene acrea(Drury) fed cotton leaves (0.0). Direct comparisons revealed thatH. Virescenslarvae fed okra leaves were about half as attractive as an equal weight of okra leaves andH. Virescenslarvae fed okra leaves and artificial diet were 1:6 times more attractive than an equal weight of larvae of the same species fed only artificial diet. This is the first report that a volatile attractant, obtained when the host feeds on an attractive plant, causes an otherwise unattractive host to become attractive to its parasite.