Seeds of three C₃ and one C₄ annual weedy species were collected from agricultural fields in Beltsville, Maryland in 1966 and 2006, when atmospheric CO₂ concentrations averaged about 320 and 380 mmol mol^-1, respectively. Plants from each collection year were grown over a range of CO₂ concentrations to test for adaptation of these weedy species to recent changes in atmospheric CO₂. In all three of the C₃ species, the increase in CO₂ concentration from 320 to 380 mmol mol^-1 increased total dry mass at 24 days in plants from seeds collected in 2006, but not in plants from seeds collected in 1966. Shoot and seed dry mass at maturity was greater at the higher growth CO₂ in plants collected in 2006 than in 1966 in two of the species. Down regulation of photosynthetic carboxylation capacity during growth at high CO₂ was less in the newer seed lots than in the older in two of the species. Overall, the results indicate that adaptation to recent changes in atmospheric CO₂ has occurred in some of these weedy species.