Postemergence control of common lambsquarters with glyphosate in glyphosate-resistant crops has generally been effective. In 2002, a common lambsquarters biotype from Westmoreland County, Virginia was identified after not being controlled with a postemergence glyphosate application in glyphosate-tolerant soybeans. Mature common lambsquarters plants were harvested from the field where unsatisfactory control was observed, and two generations of progeny from one of these plants were screened for sensitivity to glyphosate. Because the population would not have been challenged with glyphosate in the grower's normal rotation in 2003, no collections or observations were made in this growing season. In 2004, mature plants from this field which survived glyphosate application were harvested, and two generations of progeny from these plants were also screened for glyphosate sensitivity. All comparisons were made relative to a susceptible biotype from Montgomery County, Virginia. The 2002 Westmoreland and Montgomery F1 biotypes were controlled 73 and 97%, respectively, at 28 days after treatment with 1.12 kg/ha of glyphosate. Control of three 2004 Westmoreland F1 biotypes ranged from 66-85%, compared to 90% for the Montgomery biotype. Common lambsquarters susceptibility to glyphosate varied significantly among F2 lines. One Westmoreland F2 line exhibited only 68% control when treated with 2.24 kg/ha of glyphosate compared to 100% control of the Montgomery biotype. Sequential applications of 0.56, 1.68, and 2.24 kg/ha of glyphosate were made at three week intervals to 400 individual seedlings from this F2 line, and to 400 individual Montgomery seedlings. Vigor reduction at three weeks following the third application was 61.0 and 99.7% for Westmoreland and Montgomery biotypes, respectively. Of the 400 plants treated, 350 Westmoreland lambsquarters seedlings survived the sequential application, where 9 Montgomery seedlings survived this application.