Biotypes of common lambsquarters with tolerance to glyphosate have been identified in a number of states, but little is known about their fitness characteristics. Field and greenhouse studies were conducted to characterize the response of selected glyphosate-tolerant common lambsquarters biotypes to glyphosate, and also their biological and reproductive characteristics. In a greenhouse dose-response study, GR50 and GR90 values for four tolerant biotypes ranged from 1.48 to 3.22 and 8.73 to 18.7 kg ae ha-1, respectively, compared to 0.57 and 2.39 kg ae ha-1, respectively, for a glyphosate-sensitive biotype. In a field dose-response study, the GR50 and GR90 values were 0.06 and 0.48 kg ae ha-1, respectively, for a tolerant biotype, compared to 0.036 and 0.19 kg ae ha-1, respectively, for the sensitive biotype. The growth rate, time until flowering, and seed production of eight tolerant and two sensitive biotypes was evaluated in a field study. The tolerant biotypes grew taller, amassed more leaf area and dry weight, and advanced through growth stages more rapidly than sensitive biotypes during the early portion of the growing season. The tolerant biotypes were taller than sensitive biotypes at 6 and 10 wk after transplanting, but had lower dry weight at maturity. Tolerant biotypes initiated flower primordia approximately 6 to 8 wk after transplanting, whereas sensitive biotypes required 12 wk. However, no apparent fitness penalties were observed in glyphosate-tolerant biotypes based on seed-production estimates.