none The significance of nicotine in the three three trophic level interaction involving tobacco (Nicotiana tabacum), the tobacco hormworm (Manduca sexta), and the parasitoid Cotesia congregata was investigated in field plots of two varieties of tobacco which had about a 10-fold difference their nicotine content. While M. sexta mortality, rates of parasitism by C. congregata, and the total number of C. congregata larvae produced per host were similar on each two varieties, the number of parasitoids reaching adulthood on the low nicotine treatment was nearly twice that on the high nicotine treatment. This difference was due to the significantly greater proportion of parasitoid larvae which failed to emerge from the host or that died prior to pupation after emerging from hosts which fed on the high nicotine variety. A greater proportion of larvae from hosts which fed on the low nicotine tobacco died as pupae. No treatment differences occurred for either sex of the parasitoid in individual dry weight, longevity or pupal development time, except that female pupal duration was prolonged in the high nicotine treatment. These results support the suggestion that plant allochemicals, which may function to provide plant resistance against pest herbivores, can be detrimental to natural enemies of the pest.