The application of the concept of committed phloem ingestion (CPI) of aphids was studied for different aphid species feeding on resistant and susceptible plants. Greenbugs,Schizaphis graminum(Rondani) (biotype E), were monitored electronically on wheat,Triticum aestivumL., andSorghum bicolor(L.) Moench, cultivars, and Russian wheat aphids,Diuraphis noxiaMordvilko, were monitored on wheat cultivars. The duration of phloem ingestion (PI) events of these aphids was not randomly distributed and ≍53% of them were<60 min in duration. The minimum PI duration to be considered as committed feeding (threshold of CPI) was determined for each aphid–host combination. Thresholds varied depending on the aphid species and host plants, indicating the difficulty of assigning a single threshold value that could be useful as a measure of host acceptability. Also, if a given threshold duration is chosen as a criterion for having achieved CPI (e.g., 15 min), the proportion of PI events that continues for a long time period varies from host to host. Therefore, we recommend that use of this parameter be discontinued, and that measurements of the time from the beginning of the experiment to PI and the time from the initiation of a probe to PI be substituted for time to first CPI.