The Mi gene in tomato confers resistance to pests including the potato aphid, M. euphorbiae, whitefly, Bemesia tabaci , and root knot nematodes, Meloidogyne spp. A previous alternating current electrical penetration graph (AC-EPG) study with one potato aphid isolate indicated that the factors responsible for resistance were phloem-limited. In contrast, a more recent direct current EPG (DC-EPG) study with whiteflies indicated that whitefly resistance was due to factors encountered in the epidermis/mesophyll layers. The present study utilized DC-EPG to compare the feeding behavior of two potato isolates (WU11 and WU12) with different responses to resistance. Mi has very strong antixenotic effects on isolate WU11. In contrast, the semivirulent WU12 isolate can colonize and damage resistant plants, although Mi causes a modest reduction in its population growth. DC-EPG analysis of WU11 feeding behavior indicated that both phloem and non-phloem components were involved in Mi resistance, whereas phloem feeding alone was affected for the WU12 isolate. This study confirms that Mi resistance involves factors localized in the epidermal or mesophyll layer as well as phloem-limited factors, and it indicates that the relative importance of these two components of resistance differ among insect populations. The data also suggest that the ability of WU12 aphid isolate to colonize resistant plants is due to its insensitivity to deterrents encountered along the stylet pathway, and to its ability to establish a series of abbreviated feeding bouts on resistant plants.