The relationship ofHypera postica(Gyllenhal) larval age to attack by the parasiteBathyplectes curculionis(Thomson) was studied in the laboratory. The rates of parasitism of the alfalfa weevil instars indicated the first 3 are preferred and are more susceptible to attack by the parasite than the 4th. Survival of younger weevil larvae after exposure to female parasites was markedly poorer than that of unparasitized larvae. Weevil larvae died from an apparent combination of puncturing by the parasite ovipositor and effects of parasite larvae. Mortality of host larvae was greater with multiple punctures than with single.