Background and Aims: Understanding crop responses to increasing atmospheric CO2 requires knowledge of how their root systems grow, proliferate and function. The effect of elevated CO2 on the growth and proliferation of wheat root system (Triticum aestivum L.), was examined. Methods: Two pairs of sister lines of wheat contrasting in vigour (CV97 and CV207) and tillering (7750N and 7750PF) were grown in rhizo-boxes under ambient (380 μl L-1) and elevated CO2 (700 μl L-1), and the root growth and proliferation mapped. Results: Elevated CO2 effects on shoot and root biomass were observed in the lines contrasting for vigour, but not in the lines contrasting for tillering. Root biomass was reduced by 67 % in the high vigour line CV97, reducing total plant biomass by 26 % compared to the low vigour line, CV207. This was due to a reduction in root length down the 1 m soil profile and root proliferation in the top 0.2 m layer. The reduction in root biomass was not compensated by an increase in shoot biomass. Conclusions: The reduction in root biomass under elevated CO2 in the vigour line CV97 can be explained through its inability to increase the sink strength due to the failure to increase tiller number to which the plant presumably responded by increasing losses of the newly assimilated carbon by respiration. © 2013 Springer Science+Business Media Dordrecht.