Resistance mechanisms of 5 Lycopersicon pennellii (Corr.) D’Arcy accessions to Spodoptera littoralis (Boisduval) were compared with that of 2 commercial cultivars of L. Esculentum with the ultimate intent of using L. pennellii as a source of resistance to be incorporated into L. esculentum . First instars were trapped by the sticky exudate which covered the leaf surface of L. pennellii , but 2nd-5th instars overcame this physical barrier. Second and 3rd instars were influenced by an extractable substance present within, but not on, L. pennellii leaves, which had effects similar to juvenile hormone. The L. pennellii accession Sisicaya 751 had the highest apparent level of this juvenile hormone-like activity among those tested. Leaf feeding or incorporation of material from hexane or acetone extracts of L. pennellii leaves into artificial diet or application of extracts directly to larvae caused high larval mortality, adult morphological deformation, male sterility, and inhibited oviposition. Large larvae (4th and 5th instars) were not affected by this juvenile hormone-like substance. Collectively, the data suggest that certain accessions of L. pennellii could serve as sources of resistance to S. littoralis which could be incorporated into an L. esculentum background.