The pea aphid Acyrthosiphon pisum (Harris) species complex consists of at least 15 genetically distinct host races, also called biotypes. While all pea aphid biotypes live on plant species belonging to the Fabaceae, each host race shows distinct preferences for certain legume species. However, it is not yet known which plant factors contribute to the ability of different biotypes to feed on different legumes. In this dissertation, a selection of different pea aphid biotypes and legume species were used to study the localization of plant factors affecting pea aphid ability to feed on host and non-host plants. The localization of plant factors affecting pea aphid ability to feed was conducted using the the electrical penetration graph technique (EPG) which enables real-time monitoring of aphid probing and feeding behavior by measuring the fluctuations in the electrical resistance generated in a circuit set up to include both the aphid and the plant. Mechanisms that influence the ability of aphids to feed on a certain plant are discussed in the light of the behavioral data.