Six community statistics were computed to describe the structure of two plant communities and the effects of herbicide perturbation on community structure and composition. It was found that the number of individuals and the total plant height were correlated, and the three diversity components (species number, community dominance, and information content) were highly correlated. Auxin herbicides caused an increased in the number of grass with marked and persistent simplification of both community types. the nonselective herbicide, Diuron®, also induced a very simplified community, but with no change in the graminoid: forb ratio. Simazine® treatment also had long—lasting effects on species diversity, but resulted in a more complex vegetation than found in control plots with a much lower cover of broadleaf weeds. Paraquat® initially eliminated both monocotyledons and dicotyledons, but vegetation recovered rapidly. However there was a long term effect on the composition of the recovered vegetation. The resistance to modification by herbicides was greater in the old field community that in the recently disturbed one.