The effect of enhanced air CO2 concentrations (C520 and 0^50 = 520 and 650 cm^ m"^) on the growth of Lamium galeobdolon and Stellaria holostea and on the competition between the two species was examined. After five months growth imder CO2 enrichment the dry masses of both species increased when the plants were grown in monoculture, but the increase in biomass was much more pronounced in Stellaria. When the plants were grown together in competition, the measured shoot masses of Stellaria were again higher under C520 and 0^50 than at ambient CO2 concentration (C390 = 390 cm^ m'^), while the shoot masses of Lamium strongly decreased at Cgso- The effect of CO2 enrichment on the two plant species in monoculture differed significantly from that observed in mixed cultures. In terms of plant relative yield, Stellaria benefitted slightly but insignificantly from competition, while Lamium was significantly suppressed imder c^sq. Total community production of the mixed culture was optimum at C520, while that of the monocultures was highest at c^sq. At C390 and C520, growth of Stellaria depended strongly on irradiance in all types of culture. At C650 no such dependence could be demonstrated.