A chemical that stimulates oviposition by females ofEucelatoriasp. was extracted from cuticles of the larvae ofHeliothis virescens(F.) by chloroform-methanol (2:1), and was found to be active when reapplied to agar-filled extracted cuticles. Although ovipositional activity was present in both phases after the chloroform-methanol extracts were partitioned with water, most was in the nonaqueous phase. Concentration effects were important: natural levels of the reapplied stimulant gave the greatest ovipositional activity. Hemolymph applied to the cuticle inhibited oviposition and storage ofH. virescenslarvae at 15°C for 2 days greatly reduced the levels of ovipositional stimulant present on the cuticles.Eucelatoriafemales parasitized round but not flattened cuticles; host movement was not essential for oviposition.