Salinity stress may affect the response of grass species to atmospheric CO2 enrichment. But the extent and timing of growth responses in hyperarid environments may be different depending on the specific environmental conditions and the type of grass species (C3 vs. C4). The aim of the present study was to assess the extent of Cenchrus ciliaris L. responses to CO2 enrichment and to evaluate the role of salt stress in affecting such response. Three treatments were tested ; namely ambient, alternating and enriched CO 2 concentrations (ACO2 , ALCO2 and ECO2 ; respectively). Shoot length of C. ciliaris was positively affected by atmospheric CO2 enrichment. The number of inflorescence increased with the increase of CO2 level under both salt stress and control conditions. Growth allocation to green blades increased ten times under salinity stress than under control conditions. Allocation to inflorescence parts was almost two times higher under control than under salinity stress. Moreover, root allocation underwent variations between control and salinity stress. In short, CO2 enrichment did influence growth responses of C. ciliaris, especially under salinity tress. Growth allocation was also influenced toward a more reproductive growth pattern. Moreover, plants under salt stress may have responded to the CO2 alternation as an added stress.