An ecotype of Poa annua was subjected to strong selective pressure by repeated metoxuron treatments. A highly resistant biotype appeared. Under natural conditions the resistance of the biotype under selective pressure evolved more or less rapidly according to the gene flow to which it was exposed. The occurrence of herbicide resistant biotypes seems to be a general phenomenon in many weedy species. We have noticed evolution towards increased resistance to atrazine in populations of Echinochloa crus galli, Setaria viridis, Digitaria sanguinalis and Veronica persica. No such change was observed in Chenopodium album which apparently lacked atrazine resistance genes in the population studied.