Technical Abstract: In feeding, aphids inject saliva into plant tissues, gaining access to phloem sap and eliciting (and sometimes overcoming) plant responses. We are examining the involvement, in this aphid-plant interaction, of individual aphid proteins and enzymes, as identified in a salivary gland cDNA library. Here, we focus on a salivary protein we have arbitrarily designated Protein C002. We have shown, by using RNAi-based transcript knockdown, that this protein is important in the survival of the pea aphid (Acyrthosiphon pisum) on fava bean, a host plant. Here, we further characterize the process of knockdown aphids. The encoded protein fails to match any protein outside of the family Aphididae. By using in situ hybridication and immunohistochemistry, the transcript and the protein were localized to a subset of secretary cells in principal salivary glands. Protein C002, whose sequence contains an N-terminal secretion signal, is injected into the host plant during aphid feeding. By using the electrical penetration graph method on C002-knockdown aphids, we find that the knockdown affects several aspects of foraging and feeding, with the result that the C002-knockdown aphids spend very little time in contact with phloem sap in sieve elements. Thus, we infer that protein C002 is crucial in the feeding of the pea aphid on fava bean.