Summary.Observations have been made on the effects of maleic hydrazide (MH) and 2,4‐dichlorophenoxyacetic acid (2,4‐D), used singly or as a combined spray, on the vegetation of roadside verges, over a period of 3‐4 years (1958‐61). Each year, replicate plots were sprayed in spring, and assessments of growth and floristic composition of the vegetation, which was dominated by grasses, were usually made on three occasions. Treatment with MH, used singly, resulted in a general suppression of growth and flowering, particularly of the grasses. Well‐marked changes in the composition of the sward were attributable to MH, notably the decline of tufted forms, e.g.Arrhenatherum elatius, Dactylis glomerata, and the rise to dominance of the rhizomatous grasses. These changes were associated with a progressive increase of some dicotyledonous plants, although this was effectively prevented by the combined spray of MH and 2,4‐D. Two of the commonest weeds,Anthriscus sylvestrisandHeracleum sphondylium, were almost eliminated from plots treated with this combined spray for 3 successive years; a low sward, fairly free from dicotyledons and dominated byPoarather thanFestuca, usually resulted. Treatment with 2,4‐D alone, had relatively little effect on growth, flowering and proportion of the different grasses, but there was a reduction in the weed population.Les effets de l'hydrazide maléique et de l'acide 2,4‐dichlorophénoxyacétique sur la végetation