The effect of the tomato (Lycopersicon esculentum Miller) resistance gene, Mi, on the feeding behavior of the potato aphid, Macrosiphum euphorbiae (Thomas), was studied using an AC electronic monitoring system. Aphids were monitored for at least 16 h on nearly isogenic tomato lines, and we observed waveforms corresponding to the stylet pathway phase, sieve element and nonphloem phase. The time to the first probe was not significantly different between resistant and susceptible tomato lines, indicating that leaf surface components were not involved in the resistance. Also, time to the first sieve element contact, as indicated by the sieve element phase waveform, was not significantly different, indicating that aphids were able to locate the sieve element equally well on resistant and susceptible lines. However, the duration of the sieve element phase was 7- to 10-fold longer on susceptible plants than on resistant plants. The insect also made more frequent and briefer probes on resistant tomato than on susceptible tomato. These results indicate that the mechanism of resistance mediated by Mi involves limitation of the duration of the sieve element phase, and salivation and ingestion of phloem fluids associated with it.