1. Populations of an annual plant, Abutilon theophrasti, were grown at four densities (100. 500. 1500 and 4000m[Sup-2]) and two CO[Sub2] concentrations (350 and 700 μ11[Sup-1] to examine the influence of CO[Sub2] environment on density-dependent patterns of demography and reproduction. Variables measured included survivorship, proportion of plants flowering and fruiting, number of fruiting individuals, number of seeds per individual, total seed production per population, mean seed mass, and germination of seeds produced in each environment. 2. All variables, except the number of fruiting individuals, declined with increasing density, and at the highest density no individuals set seed. The number of fruiting individuals was highest at a density of 500m[Sup-2]. In the elevated CO[Sub2] environment, survivorship was significantly reduced but the proportion of plants flowering and fruiting and the number of fruiting individuals in each population all increased. Total population seed production was higher in the elevated CO[Sub2] environment at all densities. although the differences were not significant. Significant effects of CO[Sub2] concentration were observed only for population-level variables, but not for mean individual fecundity or seed size. Seed germination declined with increasing maternal density. and no germination was recorded For seeds produced at 1500 m[Sup-2]. 3. Simple models of population dynamics, utilizing difference equations, were constructed to examine potential population-level consequences of these density and CO[Sub2] effects. In the absence of a persistent Seed pool, the simulated populations exhibited damped or stable oscillations under low germination values, but displayed non-cyclic ('chaotic') oscillations or went extinct for higher germination due to the complete failure of seed-set at high density. [ABSTRACT FROM AUTHOR]