The effects of intra- and interspecific competition on growth, net photosynthetic rate (/^n), intemal CO2 concentration (cj), leaf conductance to CO2 (gi), and xylem leaf water potentials Q¥) were investigated among well-watered seedlings of the old-field species Abutilon theophrasíi. Ambrosia tri/ida, and Helianíhus annuus. In all comparisons, *F values of the target plants were unaffected by the presence of neighbors. On the other hand, revealed inhibitory effects of neighbors on A. theophrasíi and H. annuus. The above- and belowground biomass of all species was reduced in the presence of neighbors. Hence both inter- and intraspecific competition among seedlings may be important in influencing community stiucture in old-field communities.