Technical Abstract: The aphid Amphorophora agathonica Hottes is an important virus vector in red (Rubus idaeus L.) and black (Rubus occidentalis L.) raspberries in North America. Host plant resistance in the form of a single dominant gene named Ag1 has been relied upon to help control aphid-transmitted plant viruses; however, the mechanism of resistance is poorly understood. Aphid feeding was monitored using an electrical penetration graph (EPG) on the resistant red raspberry 'Tulameen' and compared with a susceptible control, 'Vintage'. There were no differences in pathway feeding behaviors of aphids as they moved toward the phloem. Once in the phloem, however, aphids feeding on resistant plants spent significantly more time salivating than on susceptible plants and ingested significantly less phloem sap. This suggests that a mechanism for resistance to A. agathonica is located in the phloem. Reduced ingestion of phloem may result in inefficient acquisition of viruses and is a likely explanation for the lack of aphid-transmitted viruses in plantings of resistant cultivars.