The soybean aphid,Aphis glycines Matsumura (Hemiptera: Aphididae), is a major pest of soybean,Glycine max (L.) Merr. Since 2000, whenA.glycines was detected in the United States, several studies on this insect have been done in different areas, but there is no report of any studies of stylet penetration behavior byA.glycines on resistant and susceptible soybean. Assessment of feeding behavior of this aphid species was compared on four resistant entries (K1639, Pioneer® 95B97, Dowling, and Jackson) and a susceptible check (KS4202) by using the electrical penetration graph (EPG) technique. Feeding behavior ofA.glycines adults was recorded during a 9-h period. The average time needed to reach the first sieve element phase byA.glycines was 3.5 h in KS4202, whereas it was 7.5 h in the resistant entries. The total duration in the sieve element phase was longer than an hour in KS4202, and only 2 to 7 min in the resistant entries. These results suggest that morphological or chemical factors in the phloem tissue of resistant plants affect stylet penetration activities ofA.glycines. In the majority of the recordings, however, the aphid stylet reached the xylem phase before penetrating the sieve element, and the time that aphids spent ingesting xylem sap was not different among all entries. Therefore, it is possible that xylem sap in the resistant entries may contain toxic substances that change aphid behavior and that affect further activities in the sieve element phase.