Technical Abstract: The soybean aphid (Aphis glycines Matsumura) is an invasive pest of cultivated soybean [Glycine max (L.) Merr.] in North America. Recent developments in host-plant resistance studies have identified at least 4 soybean aphid resistance genes (Rag1-4) and 2 soybean aphid biotypes (Biotype-I, II), defined by differential survivability on resistant soybean. We tested detached soybean leaves as a more rapid and practical test to assess host-plant resistance and biotype determinations. We used 2 susceptible lines (Wyandot and Williams 82) and 2 resistant lines (PI 243540 and PI 567301B). Resistance was tested by placing one aphid on both whole plants and a detached leaf placed within a petri dish. Various life history characteristics were measured including percent mortality, fecundity, and life expectancy. Results indicated that resistance to soybean aphid was lost using detached leaves of PI 567301B but retained with PI 243540. To test for virulence, we compared net fecundities of Biotype-I (B-I) and Biotype-II (B-II) on detached leaves of the resistant cultivar Jackson (to which B-II is virulent). We detected a significant difference in net fecundities among B-I and B-II on detached leaves of Jackson, and used predicted growth rates to determine virulence from 30 field collected individuals. No field individuals matched B-II predictions, but 4 individuals had higher net fecundities than B-I predictions (13%), and could be considered moderately virulent. Our results indicated that the retention of soybean aphid resistance in detached leaves is dependent on source of resistance.