The objective of this study was to assess the influence of toxic substances with different modes of action on a two-species system: an aphid-specific parasitoid, Aphidius ervi, feeding on the pea aphid Acyrthosiphon pisum.  The instantaneous rate of population increase (r_i) was used as a measure of population level toxic effect in this study.  The toxicants evaluated were imidacloprid, a nonpersistent neurotoxic insecticide and cadmium, a chronic pollutant with a tendency to accumulate.  We evaluated the effects of cadmium and imidacloprid to A. pisum and A. ervi because both toxicants can occur together in polluted areas where crops are grown.  Cadmium (200 or 400 mg kg^-1 dry weight soil) and imidacloprid (4 or 40 g a.i. ha^‑1) were applied to soil contained in plastic pots in which broad bean plants Vicia faba L., were grown.  Results of this study indicated that cadmium at the concentrations tested, reduced population growth rate of the pea aphid.  Imidacloprid also reduced aphid growth rate but only at the highest concentration tested (40 g a.i. ha^‑1).  Combinations of cadmium and imidacloprid had the greatest impact on aphid growth rate.  Imidacloprid alone had no affect on population growth rate of the parasitoid, A. ervi.   However, cadmium alone or in combination with imidacloprid had a negative impact on A. ervi reducing population growth rate 77%.  These results indicate that negative impacts on parasitoids may occur in areas where cadmium contamination is present and imidacloprid is used to control aphids.