The relative success of six populations of the annual Phlox drummondii raised under conditions of intraspecific competition was studied in five field sites and an experimental garden located in central Texas. All pairwise combinations of seedlings from six Phlox populations and pure stands of each were planted at a density of eight plants per styrofoam cup. The cups were taken to the field sites, the bottoms were removed, and the cups sunk into the soil. Five replicates of each population pair were transplanted into each of the six sites from which the seeds had been collected. After the flowering season the pots were collected and the number of flowers (or fruits) and amount of biomass per plant were determined. Flower production was strongly related to biomass (° =0.96, P<.0001). In general, pure stands had higher mean flower production than mixed stands, and in mixtures there was a significant tendency for plants grown at their native site to outcomplete aliens in most sites. The production differential between natives and aliens was greater in mixed stands than in pure stands. No one population performed best in all sites and all populations performed best in all sites and all populations performed best in the experimental garden. The results indicate that Phlox drummondii is locally adapted to its microenvironment. The specific outcome of competition is dependent on the site in which competition occurs and the orgin of the competitors.