The effect of parasitism by Aphidius ervi Haliday (Hymenoptera: Aphidiidae) on development, survival and reproduction of pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), of different ages was studied in the laboratory. Aphids parasitised when 1 and 2 days old (1st and 2nd instar) died at the 4th instar. However, those parasitised when 3 to 6 days old (3rd and 4th instar) could reach the adult stage following parasitism and those that were parasitised after 4 days old (late 3rd instar) were still able to produce progeny. In comparison with the unparasitised aphids, the parasitised aphids had a significantly shorter reproductive period and produced significantly fewer progeny, and thus had significantly lower intrinsic rates of increase, net reproductive rates, shorter generation time and longer doubling time. The potential impact of the parasitoid on host population growth is discussed. none